Post your addons here, might be usefull for the rest of the guild.
I'll start with a screenshot of my interface and explain my addons.

main addons are:
Ace2
Armory (view a players armory profile ingame)
Atlas (dungeon maps)
bartender (better usage of buttons)
battefieldcommander (see what's going on in a battleground)
buffalo (better way to sort buffs)
cartographer (X Y locations etc)
eepanels (create panels for look and feel)
enhanced tooltip (better tooltips)
equipcompare (compare a item to your equipment, very handy for loot or auction house)
fubar (handy bar for addons and various information like FPS and such)
groupcalender (for the guild, know when a raid/dungeon or whatever is about to happen) !!! MUST HAVE
ignoreduel (.. i don't like to be dueled outside a battleground)
omen (threat meter, good if you do alot of dungeons)
pitbull (configurable frames, like character and pet's, target etc)
quartz (casting bar addon)
sct (scrolling combat text)
skinner (add some creativity to your interface)
SWStats (Report damage, healing done and much more)
XLoot (I find this handy, no more moving mouse to loot)
zChat (simple, you can scroll with the mousewheel to go up [or down]. )
So, what are your addons?