Addons
I’ve put together a list of all the addons that I am currently using or that I have used in the past. I don’t use all of these on every profile. For example, I don’t use BankItems on every character, I usually only use it on my bank toon; Altoholic I have on EVERY toon; and Clique I don’t use anymore since I have my new toy.
General
Advanced Trade Skill Window - Much easier to use than the standard UI for Trade Skills
Altoholic - Handy little mod to keep track of all those alts out there…and their professions, mail, bags, bank, etc.
Atlas - I don’t use Atlas or Atlas Loot unless I have a toon that is in the high 40′s becuase I don’t really need the maps/loot info until then. And at that point I’m quest happy and don’t pick my dungeon want’s based on the loot inside.
Auctionator - I like this better than auctioneer and it’s smaller.
Bagnon - Similar in funtion to OneBag but is being updated with frequency.
BankItems - I don’t use this anymore as I can see what is in the bank on all my toons using altoholic.
Bartender4 - UI Bar Mod
BugMeNot - Handy if you’re like me and don’t raelly like random pesky group invites and duals.
Carbonite - I use Carbonite with TourGuide. Carbonite has a great MiniMap Button feature that you can move and change the columns etc. I would use carbonite for that feature alone but since it comes with a great map and quest info…bonus.
Cartographer - Now that I have used cartographer I get frustrated when I load up toons that I don’t have it on. Great Map addon.
Decursive - Exactly as it sounds but way more handy on all types of toons.
Elkano’s Buff Bars - Took me forever to figure out how to turn it from standby to active but once I read the directions, as I should have from the start, I can’t live without it.
GoingPrice - Nice to have for prcing the going price and average cost of items especially if you use the AH.
LeaveMeAlone - Similar to bug me not but they each have different featuers. Pick one and use it. They won’t work together.
LightHeaded - Very nive mod. Has WOW Head info in a frame attached to the quest log. I used to not be able to tab out a lot or my computer would not like WOW for a while. Even now that I play in windowed mode (Mac) I still like to have this on my toons.
Lunar Sphere - I use Lunar Sphere along with Bartender and Xperl. Why? Becuase adding my own speeches to the auto speech function ROCKS! Aside from that, I am able to put all my potions, food, scrolls, bandages on the click buttons that you can open and close without cluttering my bars and taking up valuable space on screen.
Omen Threat Meter - Exactly what the name suggests. I use this all the time with my hunter so that I can make sure if I’m using my pet to tank that I don’t surpass his threat. Handy for the MM spec so I know when to stop over damaging my pet. 
Postal - I like postal because it lets you hit an Open All button on your mail UI and open everything at once. Saves lots of time.
Prat 3.0 - Chat Box Mod – Love it’s many options including a free floating edit box.
Quartz - Cast Bar Mod.
Recount - I only use this to see how much damage I’m putting out on Mobs and see if it is where I should be in regards to DPS and Class info.
TargetCharms
TomTom - Way Point arrow.
Visual Heal - Gives you a visual bar to show how much you are healing or overhealing.
WIM - Instant Messenger like windows for whispers.
X-Perl – Player unit frames.
Druid
Hunter
AutoAspect - Pops up a message when you are low on mana. Has handy button that comes up in the middle of the screen you can just click on. Also has a message when you are near full to switch back.
Auto Track - Changes your tracking to the mob top you have targeted.
Nudge - Range indicator. Simple and light, moveable.
Paladin
Seal Countdown - Pop up text reminder when seal is about to expire. 
Hasn’t been updated for the latest patch and is currently not working for me.
Arena/Battlefield/PVP
