I like to tinker with stuff, and games that allow even a bit of customization can entertain me. World of Warcraft offers more than just bit of custmizations, and I often find myself playing with my UI more than actually playing the game. So while probably not needed I thougt I would highlight some of the mods I have encountered, and sites that host mods.

First the sites I most often visit when looking for mods:
ui.worldofwar.net
www.wowinterface.com
wow.curse-gaming.com
www.wowace.com
auctioneeraddon.com
Blizzard UI forum

Some of my favorite mods:
MetaMap
FuBar and its plug-ins
Bongos
Ag_unitframes
zhunter
Enhanced Casting Bars
SCT and SCTD
Monkey Quest