The Yu-Gi-Oh abridged series is comedy gold.
I don't really watch other abridged series', but PokemonAbridgedFTW was pretty good. Before the channel got suspended. ;_;
Agreed. MasterWGS and his team have a website, though, dunno if you knew, but in case you didn't they're still making videos, I think.
I got started with Yu-Gi-Oh! Abridged after hearing a lot of references to it on the web. Besides that, I've also watched:
*TeamFourStar's Dragon Ball Z Abridged
*Yu Yu Hakusho Abridged (shame it hasn't updated recently)
*Pokemon Abridged
*The Abridging of Haruhi Suzumiya
*Naruto Abridged
And... that's about all, I think.