I'd like to think that this would be the full list of NPCs that can be married in the game. However, Skyrim is a very big game, likely not all NPCs you can marry have been located yet.
http://elderscrolls.wikia.com/wiki/Marriage

I was just curious because if you'll notice, there are no male Khajiit in Skyrim you can marry (as of the list on Nov. 13th, 2011).

Makes me sad, hopefully there will be some cute ones that turn up.

EDIT: Double checked, looks like no Khajiit want to marry in Skyrim.