There's a few different kind of clean versions.

Different vocal versions:
Ice Cube - AmeriKKKa's Most Wanted
http://www.youtube.com/watch?v=VuGgD-eoCDM

Silenced or backward masked versions
N.W.A F**k Tha Police Clean
http://www.youtube.com/watch?v=9306jTZuU5k

Sound effect'ed versions:
http://www.youtube.com/watch?v=HxwlyV8QxZ4


I still say this is one of the worst ones:
Wu-Tang Clan - Shame On A Nuh
http://www.youtube.com/watch?v=vK2tOCSWf6M

Anyone got outstanding examples?