Google Wave: View all public waves
Search for with:public and you’ll have enough to keep you busy testing.
Update: To make your wave publicly viewable, add this “firstname.lastname@example.org”
Note: You have to add it in the contact list on the bottom-left of the screen. It might not accept the email address, but just press enter anyway and it should work.
Note2: Sometimes the address will disappear from your list – in which case, add it again (I know, it’s annoying).