Ok, so I am somewhat new to WoW, but I love my addons (truth be told I love any sort of gadget/accessory type item). Through various searches and guild chat monitoring, I've found quite a few useful ones. How about we share any other gems that might be useful.

I'm particularly lost when it comes to addons for healing / buffing / raids. I've installed a few for this purpose, but haven't quite figured them out yet.

My Addons:
Accountant - I love to see how much money I'm spending
Atlas - Instance maps
Auctioneer - The AH is an addiction. Knowing what something is worth while adventuring is great
BankItems - Allows you to see what is in the bank and also for your alts
CharacterProfiler - (RPG Outfitter)
CooldownCount - Puts a cooldown timer on top of spells/items in your toolbar
CT_MailMod - Mass mail (send multiple items with ease)
EquipCompare - Compares any item you mouse over to your currently equipped gear
FishingBuddy - Has a 'quick recast' which rocks
Flightmap - shows time to destination for those 15min grypon flights
Gatherer - tracks resource spawn points
MobInfo2 - keeps track of hit points / drops / etc for mobs you fight
MonkeyQuest - nice quest manager
ntmysFixLoadingTimes - speeds up loading of instances / etc
PriceEach - at the AH, shows you a 'per item' price
QuickLootImproved - auto-moves the loot window to under your mouse and auto-closes empty loot windows
QuickMountEquip - switches to riding gear when you mount
Recap - damage meter
SomeAssemblyRequired - shows all of the base mats needed to make items
Titan Panel - status panel which has many different things (bag slots, coordinates, money, repair status, xp, etc)

Addons I have installed but haven't figured out yet:
BuffAhoy
CT_RaidAssist / CT_RaidTracker
Decursive

For Decursive just make a /decursive macro...that's it!

Even better, for decursive type

/dcrshow

and it will bring up a little icon/window on your screen you can move whereever you want. Whenever someone in your party gets nailed with something you can fix (as a paladin this would be poison, disease, or magic debuff like frost nova), it will show an icon and the name of the person in the list. Just click the icon and you'll automatically cast Cleanse on that person.

good post by the way, I use a lot of mods, I'll post them all tonight.

I've spent some time configuring CT_RA. It's useful, but it takes effort to get it right. I've also spent the last two days tweaking Discord Unit Frames (http://www.discordmods.com/cgi-bin/page.pl?mod=DUF). Again, a super great mod, but it takes a lot of config time.

I'll put up a screen shot of my new UI later.

I enjoy a couple of mods such as:

AutoProfit - Gives a button that automatically sells vendor trash
AutoRepair - When you talk to a vendor that can repair, it will automatically put up a window prompting you to repair, and it includes a "fix all inventory" feature.

Several others that I can't remember the names to right now, one is neat for rogues, it gives a button in the trade window to unlock boxes quickly and also allows you to open boxes by right clicking on them in the inventory.

Alright, here's what I am using:

Ace 1.3 (several other addons below depend upon this library)
AceGUI (same as above)
Acuzod's Available Only (when talking to a trainer of any kind it only shows what skills I can learn at that time)
AH Wipe (clears all search parameters on the AH screen)
Art Hider (hides the dragons to the bottom left and right to allow more room for buttons and the background)
Atlas (in-game maps of all instances and dungeons including boss locations)
Auctioneer (allows you to "scan" AH, takes 15-20 minutes, and compiles a report database you can search for good deals, bargains, and provide comparisons when you put your own stuff up for sale)
AutoProfit (click a single button when at a merchant to auto sell all greys in inventory)
BlindFlashCount (rogue add-on that shows on the Vanish button how many flash powders you have left)
BonusScanner (scans equipment for item bonuses and provides a cumulative report)
BookOfCrafts (database that keeps track of what all recipes are known by you and your alts so you know what you need to purchase)
CCWatch (provides a progress bar of CC on-screen so you know how long a CC has remaining)
ChatLink (allows you to link items in private chat rooms)
ChatScroll (makes the chat window mouse wheel scrollable)
ChatTime (timestamps chat messages)
CT_BuffMod (repositions buffs and shows duration and name)
CT_ItemBuffs (shows item buffs, like sharpening stones, in the buff menu and the time left)
CT_MailMod (allows you to send multiple items to one person in a single email)
CT_MapMod (allows you to make notes on the world map)
CT_MasterMod (library for the above and below mods)
CT_PartyBuffs (shows more buffs at party frame)
CT_RaidAssist (allows you to keep track of all members of a raid easily not just your own group)
CT_UnitFrames (allows you to display hp/mana values and/or percentages in party windows)
Decursive (easy way to cure and monitor ailments that your character can cure on all party members)
Enchantrix (database that shows the percentage likeliness of what you'll get when you disenchant an item)
Enhanced Flight Map (flight database that tracks the amount of time it takes to get from one location to another)
EnhToolTip (shows more information when hovering over items)
EquipCompare (automatically compares items you hover over to your own equipment)
EzDismount (automatically dismounts from horse if you try to mine a vein or pick a flower or something)
FriendsShare (keeps track of the Friends list between all of your characters so you don't have to reset it for every alt)
Gatherer (keeps a database of resources found across the world, for instance ALL mines you mine from, where they were located, etc.)
Informant (part of Auctioneer and EnhToolTip)
KC_AutoRepair (once configured, anytime I talk to a merchant my equipment and inventory are automatically repaired and a report showing how much it cost is placed in the chat window)
KC_Items (database of what professions or tasks various found items in the game may be used for)
MetaMap (enhanced world map, windows rather than fullscreen)
MetaMapFWM (part of MetaMap which shows unexplored areas a different color)
Mob-Info2 (shows more information on a mob like estimated HP when you hover over them)
ntmys FixLoadingTimes (gift from Rinader, it GREATLY reduces load times for instances and zoning)
OutfitDisplayFrame (a frame to display sets of equipable items while in other windows like AH)
QuestAssist (keeps track of quests you are working on more easily)
QuestLevel (shows the level of a quest in the quest log)
QuickMountEquip (changes my equipment I'm wearing when I mount a horse to include mithril spurs and carrot, and changes back to normal eqiupment when I dismount)
RareTracker (keeps track of all possible rare spawns in the world, showing what all are possible in your zone, and scans for them nearby)
RareTracker Map Support (part of RareTracker)
RareTracker Standard UI (see above)
RareTracker TitanPanel UI (see above)
Recipe Radar (searchable database that lets you figure out what all vendors in your area sell recipes that you or your alts don't already know, whether you can learn them, etc. VERY good for finding recipes you're not sure of.)
RecipeBook (keeps track of what all skills and recipes you and your alts know)
rpgo-Character Profile (addon for RPGOutfitter.com)
SmartHeal (I haven't set this up yet, but it's for automatically selecting appropriate rank of a spell to conserve mana)
Stubby (part of Informant and Auctioneer)
Telo's LootLink (in game browsable database about loot, what mobs it comes off of, etc.)
Telo's MobHealth (keeps a database of creature health, etc.)
Telo's QuickLoot (automatically positions the loot screen under your mouse cursor)
Telo's SelfCast (hold Alt while clicking a spell and it automatically is cast on you without losing your target)
Titan Panel (very good add-on for displaying a ton of information in game in a narrow bar such as your gold, durability, location coordinates, etc.)
Item - All-In-One-Inventory (this one combines all of your bank and bags when you click on them into one MUCH larger window combined, so you don't have to open all bags one by one).

Wow that was a lot of typing O_O

EDIT: Make damn sure you set your script memory to at least 64 MB to do this :)

I'm pretty much only using:
Atlas
Auctioneer
CT_RaidAssist
enchantrix
enhanced tooltips
Equipment Compare
Gatherer
Lootlink
Mobinfo2
Scrolling Text - higly recommend. damage and abilities flash over your head. Final Fantasy style
Titan Panel
ntmysFXLoadingTimes - super fast load times.

I have a few of the "standard" mods installed, but my only must have is:

CupHolder - keeps your beverage cold while playing

The big ones I use that Jami didn't list (we share an interface directory) are Discord Action Bars (I HAVE ALL THE BUTTONS IN THE WORLD), Discord Unit Frames (everyone's health bars are HUGENORMOUS), and Discord Frame Modifiers (also now they are pretty).

Also I have FishPing, which err makes the fishing bobber sound more noticeable so I don't lose a fish. I AM A FISHING NERD OKAY DON'T JUDGE ME.

I am trying to find the patience to be a fishing nerd...maybe the FISH PING would help!

As promised, the UI shot.

http://iwhitney.hopto.org/~inky/vs/button_master.jpg

I am the button master! So many buttons!

Basically I have the entire game mapped to my keyboard. So I may make those buttons a lot smaller after I try this UI out a bit. Notice the handy (and large) health % that appear next to people's health bar. That is so you do not die.

That's pretty much exactly what I've got going on, too -- separate bars for potions, damage, heals, and buffs. I had not considered mapping to the keyboard, but that would certainly help my not-casting-flash-in-time problem. I'll take a shot of mine tonight.

YMMV, but here's what I've got.

Ctrl+Shift 1-5 targets party members 1-5
Ctrl+Alt+Shift 1-5 targets party pet members 1-5 (I'll never have a pet, but it's still mapped)

Ctrl 1-6 are my basic heal spells.
Alt 1-2 are my basic de-debuff spells.

I have the mappings set up so that I can use the number-pad numbers or the regular numbers. I mostly use the number pad.

I also set the * button on the number pad to assist. So after I cast a heal, I can quickly assist them.

So I basically spend the entire game using Ctrl, Alt, Shift and the number pad.