Daniel Carr, designer of the official 2001 RI and NY quarters, has designed 14 parody state quarters. These are available for sale. More information is at Designs Computed web site.
"Girlie" quarters for all 50 states are at Sexy Quarters.
A nice site that shows all the official designs, early designs, and in some cases designs submitted by the general public is Quarterdesigns.
Fark has had many photoshop contests for alternate quarter designs.
The Physics Geek has 11 funny quarters at the very bottom of the page.
Alien Quarters sells alien, Halloween and Santa Claus quarters.
M. John Fayhee's article "Playing Quarters in the West" in Mountain Gazette, July 2008.