Their site is a little hard to navigate, but to make things easier, just click on the WoW Ace Updater on the left there… or actually, to make it even easier, just download it here. Then run it and see all the cool addons you can pick!
Must haves are FuBar, Cartographer, either agUnitFrames or PitBull, and, if you do raids, oRA2 and Big Wigs.
What’s funny is that two different people recommended the Ace Updater on the same day. Hooray for viral networks!