here are a few really amazing collections dealing with rare, crested, variegated, monstrose cacti etc:
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.
insane, has to be seen to be believed.
hundreds of awesome pages on this site. could spend all day browsing through.
really rare stuff: crests, monstrose, hybrids, chimeras, variegates etc.
one for ario fans.
and if you haven't seen it already this is quite possibly the rarest cactus on earth:
Astrophytum caput-medusae (Digitostigma caput-medusae) chimera with myrtillocactus geometrizans, owned by the actual discoverer of A. caput-medusae.
add more links!