snowy listed a bunch of other UI mods which do a variety of things you may or may not need. If you look them up on curse (
Curse.com - WoW Addons WAR Addons Gamer Blogs Forums), you can figure out if you want them.
Wowhead (
Wowhead: NOM NOM NOM), on the other hand, is an essential all-purpose WoW resource. It has every item in the game (and a few that aren't anymore), every NPC, every quest, every...well everything. Talent calculators, pet talent calculators, spells, achievements, professions.
If you're trying to find something and you don't know where it is or where it's from, go there and type it in (or better yet get the firefox search plugin) and chances are much better than not that it'll pop up. Use it, love it, and never feel confused agian.