Vixtorinox - the 58mm SAKs are excellent keychain knives. They range from the small Signature to the chunky MiniChamp.
Spyderco - The Ladybug would be my choice from Sypderco, although the Spin and Byrd Finch are good choices, too.
Kershaw - Their little Two Can knife/scissors is a unique keychain knife, or their aptly named Key Chain knife.
Leatherman - They have several keychain sized multitools, including the Micra and their Squirt line.
Buck - they make an interesting little keychain knife called the Metro in several different colors.
Benchamde - makes a nice little knife called the Benchmite in both manual and auto.
Hope that helps!