the Kabar dozier folders are pretty slim, super light, good locks (lockback) and AUS 8 steel I believe. Also very inexpensive. I bought one for a backup to my backups.
Lots of good knives mentioned in this thread, but I have to second this suggestion. The Ka-Bar Dozier series knives are thin, come razor-sharp and lock up securely. And I think all -- including the largest -- can be had for under $30.