The addons I use are:
Align (type /align to make a grid on your screen for positioning addons)
Aloft (custom nameplates)
Auctioneer Complete Package (Auction House and tooltip improvements)
Bartender4 (action bars)
BigWigs (boss mod)
BTex (panels for bottom of screen with nice skins)
ButtonFacade and all extra skin packs (skins for bartender and buffs etc)
Clearfont2 and extra font pack (change fonts)
Cowtip (nice tooltip mod)
DoubleWide (quest window double wide)
ErrorMonster (eliminate/redirect error msgs)
FlightMap
Gladius (arena unit frame)
Grid (raid frame)
HandyNotes (put notes on map)
HandyNotes_Guild
Lagbar
LightHeaded (quest helper)
MagicRunes (dk addon)
Mapster (better map)
MiksScrollingBattleText (scrolling combat text)
NeedToKnow (bars/timers for whatever you specify)
Omen (threat meter)
OmniCC (cooldown counter on your buttons)
oRA2 (ct_raidassist like mod)
Pitbull (unit frames)
Quartz (cast bar, swing timer etc)
RatingBuster (item level on tooltip)
Recount (damage meter and much more)
SatrinaBuffFrame (best buff frames)
SexyMap (makes your minimap sexy)
SharedMedia (more textures/fonts etc)
SharedMediaAdditionalFonts
Talented (Changes Talent Point and Glyph and allows importing/applying directly from the web)
TomTom (gps)
TrainWhistle (tells you who dropped the annoying train so you can kick them from the raid)
Xloot (Customizable loot window)
Xloot_Group (Customizable group loot window)
XRS (create bars for pertinent raid information)