Queen and their various subsidiaries, German Eye, Bulldog, then there are tons of manufacturers that are either out of business or have moved offshore. Schrade, Camillus, Keen Kutter, Buck(most are China made now as I understand it, but still of decent quality from what I hear), Imperial, Hen&Rooster, etc. etc...
Seems to me that slipjoints don't have as many quality issues as "tactical" knives no matter who the maker is they're usually pretty decent. That could have something to do with the target market being a little more discerning or picky about things, or just conjecture on my part. With that said, I can't think of too many makers that you could go wrong with, but there are plenty good ones that I'm sure I've left out. Hopefully someone else will come along with a more thorough reply.
Gautier