Here are some I use:
Great site for vintage arctic cat specs:
Curt's Vintage Arctic Cat Page
Same page for kawa's:
Curt's Vintage Kawasaki Page
A lot of good articles, literature, manuals, dig around there is a lot of information/manuals behind each link:
Vintage Snowmobile Literature
Can't forget our forum friend, nice how-tos and analysis here:
Dead Sled Wrenchers
Lastly I love this page to identify sleds, this has tons of old snowmobile brochures, it is hard to use but has fantastic information on the page:
VINTAGE SNOWMOBILE BROCHURES - ON THIS PAGE YOU WILL FIND OVER 400 VINTAGE SNOWMOBILE BROCHURES POSTED.
