This ones going to be VERY individual but could we add to this a list of useful mod/addons names and what they do?
Ill start the ball rolling:
enchantrix: shows in the GUI on a mouseover what any item in its (rather comprehensive) database will dis-enchant into (inc perchantage if more than one option) god-send for enchanters
titanbar: adds a bar to the top of the screen with current pvp honour, exp per hr/time to level, bag slots left, current cash, FPS readout etc, current x,y co-ordinates as well as adding X,Y co-ord to the map. I cant live without this one now
recap: gives a very detailed breakdown of your most recent combat which (once you understand it) you can analyse for improvements in combat tactics. Gives your characters OVERALL DPS (so including buffs, use of skills etc not JUST the weapon dps) so again - you can watch for improvements etc
lootlinker: lets you know what the item is worth to sell (to vendors) and which classes can use it
auctioneer: takes a little time to get it working as it only get info from you standing at the auction house and scanning the auction, once the database is made tho will tell you recent prices etc for items (on mouseover like lootlinker and enchantrix) and suggest a good bid/buyout value
As a general suggestion tho IF and when insomniax compliation is ever updated GET THAT!!! Is MUCH more comprehensive than almost anything else available, includes an awful lot of VERY good addons. Im currently using a version of the 1.7 beta 4 release (were on 1.81 in game now so out of date) that i've played with to get working and put up with the odd error from it