I figure a mention here might be useful as well:
gamesieve.com (
just announced) has
a filter for "games with a demo", and if you toggle off grouping, also
a filter for the "demo product type" (both using a liberal definition of "demo", including prologues and various other free "taster" products for a full game).
I'm categorizing products manually; so if you disagree with me on any categorization - or if I missed anything - I'd be happy to hear about it, and could be convinced to change my ways. :)