Just a quick question; by "work" do we mean in the regular plugin directory, or the mu-plugin directory? How much does it matter?
As an example, the MyMooMus worked OOB in the regular plugins directory. It adds mood /reading / listening to tags to each post, configurable in the admin panel for each user, giving the entries a LiveJournal feel (but without the angst).
The Twilight auto-save worked OOB, but I didn't try it in the mu-plugins directory. Handy for dolts like me who navigate off the page before hitting publish. (Beat that, where'd-my-post-go-Blogger)
I also got WP Flickr Post Bar to work with a small edit specific to MU. I'll have to go check and get back to you on that one. This is handy for posting flickr-hosted photos to member blogs.