The Etoile Menu System...
Not that I dislike WildMenus, but this one just doesn't flicker on application change. ;-) And after all, it's based on WildMenus, too.
Get it here!
There's also an interesting discussion about it on the etoile-dev mailing list.
And, speaking of Etoile mailing lists, I remember seeing a post on etoile-discuss that points to NEPOMUK, a project which is currently running at the DFKI, where I got a programmer job (Hiwi). It's cool that the word of the semantic desktop is finally spread now. :-) Not only Leo will like this.