Here are the add-ons I use along with urls where you can obtain them:
Atlas mod (adds a button to the side of the minimap where you can click on it and get detailed maps of instances, battlegrounds and outdoor raids)
http://ui.worldofwar.net/ui.php?id=400
Auctioneer (erm, the name should give it away really, helps you to sell stuff and undercut competitors on the in-game auction house etc..)
http://ui.worldofwar.net/ui.php?id=137
Auto-Ding (It's a mod that can automatically spam guild chat with ding messages when you ding! Default setting is it auto-dings every 5 levels, but can be changed to auto ding every level).
http://ui.worldofwar.net/ui.php?id=3581
Capping (Ace2 Mod - uses Ace2 programming library, link on down a bit. This mod puts up timer bars on screen to let you know how much time is left before a graveyard in Alterac Valley or a flag in AB before it is captured by either side).
http://files.wowace.com/Capping/Capping.zip
Cartographer (Ace 2 Map mod, use the /way command to pinpoint co-ords and what direction, distance they are in, also shows where guild members are etc and a whole heap of other stuff, too much to mention really, so look at the link).
http://ui.worldofwar.net/ui.php?id=4008
Chat timestamps (tiny little mod that adds a timestamp to the chat window - ct_core part of ct_mod can also do this function)
http://ui.worldofwar.net/ui.php?id=3905
Crit Record (adds a record of your highest critical hit the tooltip of your attack and spell icons)
http://wow-en.curse-gaming.com/downloads/details/6044/crit-record-continued/
ct_mod (only use the ct_raid add-on of this mod, and disable the rest, apart from the ct_core part of the mod obviously)
http://www.ctmod.net/media/packages/Advanced.zip
DeclineDuel (Ace 2 mod, does what it says on the tin, it declines duels. Can be configured to send a whisper to person wanting to duel with you saying "I do not wish to duel". And can also be switched off if you do wish to duel using the slash command /dd )
http://files.wowace.com/DeclineDuel/DeclineDuel.zip
Enchantrix (part of auctioneer, useful for enchanters as it keeps track of what shards, dust etc you might get when you disenchant an item)
http://www.auctioneeraddon.com/dl/?dl=Enchantrix/enchantrix-3.9.0.1560.zip <-- stand-alone
http://www.auctioneeraddon.com/dl/?dl=AuctioneerComplete/auctioneerpack-4.0.1.zip <-- Auctioneer full, comes with everything
http://www.auctioneeraddon.com/dl/
Extended QuestLog 3.7.1 (better quest log ui, also able to track more quests on screen)
http://ui.worldofwar.net/ui.php?id=1676
FuBar! (Ace2 - better than titan panel imo, easiest way to keep fubar updated and find new plugins is to use the ace2 updater, which I will link to later)
http://wow-en.curse-gaming.com/downloads/details/3040/fubar-2-0/
Also curse gaming has quite a number of fubar add-ons listed - http://wow-en.curse-gaming.com/downloads/search/?cat=1&q_labels=1&q=fubar
Grimoire Keeper (Warlock specific mod, keeps track of all the grimoires you have for each of your warlock pets)
http://ui.worldofwar.net/ui.php?id=2542
Lib: Ace2 (the ace2 programming library, some of the ace2 add-ons depend upon this)
http://files.wowace.com/Ace2/Ace2.zip
MobHealth3 (Ace2 mod - Estimates a mobs health)
http://wow-en.curse-gaming.com/downloads/details/3619/mobhealth3/
SpamSentry (Ace 2 mod - Essential mod for blocking gold spammer whispers and for easy one click reporting of gold spammers and bots to blizzard GM's)
http://wow-en.curse-gaming.com/downloads/details/4150/spamsentry-anti-goldspam/
Phew thats the lot, the add-ons I listed as ace2 can be updated very easily with a few clicks using the Ace2 updater program: http://www.wowace.com/wiki/WinAceUpdater
or you can do it manually from this page: http://files.wowace.com/ :P
Anyone got any I havent mentioned and they find useful? Also any class specific mods that you find useful say for priest, paladin etc?
and theres me thinking 4 add on was enough