My understanding is that most plugins should work with WPMU. Most are not officially tested with it, but generally will work. So you have to try them out to be sure. I haven't had any problems so far. If you put them in the mu plugins folder vs the regular plugins folder, they are supposed to be automatically activated for all of the sites, vs having to activate them manually.... but certain plugins don't work well from the mu plugin folder, so you have to put those in the regular plugins folder. I hope that made sense. From what I can tell, you have to pay the premium membership fee to get access to the official wpmu plugins.... although I'm sure they have some on here somewhere.