here are a few really amazing collections dealing with rare, crested, variegated, monstrose cacti etc:

http://www.cactus-art.biz/
start it off with a classic everyone knows. you could send all day admiring the site, plants and photography.
cactus nursery in Italy specialising in rare forms, cultivars, abnormal forms of cactacae.

http://s8.photobucket.com/albums/a12/bukowski32/?start=all
insane, has to be seen to be believed.

http://www.lapshin.org/cultivar/index-e.htm
hundreds of awesome pages on this site. could spend all day browsing through.

http://www.lapshin.org/cultivar/N40/italy-e.htm
really rare stuff: crests, monstrose, hybrids, chimeras, variegates etc.

http://www.lapshin.org/cultivar/N25/ario-e.htm
one for ario fans.

and if you haven't seen it already this is quite possibly the rarest cactus on earth:
http://www.lapshin.org/cultivar/N36/dig-e.htm
+Digitomyrtillocactus.
Astrophytum caput-medusae (Digitostigma caput-medusae) chimera with myrtillocactus geometrizans, owned by the actual discoverer of A. caput-medusae.

add more links!