You can get very unique passport stamps in some unusual and unique places.
Unique passport stamps I've collected.
Tapuaetai, One foot island, Aitutaki (atoll), Cook Islands 2009http://en.wikipedia.org/wiki/Aitutaki
Llanfairpwllgwyngyllgogerychwyrndrobwllllantysiliogogogochl, Anglesey (island), Wales, UK 2006http://en.wikipedia.org/wiki/Llanfairpwllgwyngyll