Google has officially opened Knol, its answer to Wikipedia, for contributions from the general public. The principle innovation of Knol is that contributors will be encouraged to use their real identities, and primarily contribute on subjects within their own expertise. There are also apparently tools explicitly designed for collaboration.
Knol is still very much a work in progress: there are knols (= "units of knowledge," natch) on "Leadership 101" and ganglion cysts and how to write a knol; but, as of right now, no hits on a search for "evolution," "bicycle," "Mennonite," or even "plant." Whereas Wikipedia certainly has extensive entries for each, probably including exhaustive lists of references to the terms in the films of Martin Scorsese.
For more, see the Official Google Blog and coverage by Wired.