I carry a spare 3"(?) kitchen paring knife in a belt sheath. No springs, no hinges, no nothing and it is relatively cheap, although it is a good brand with a good stainless blade. It stays razor sharp. Doesn't get in the way. And no, it doesn't get used in the galley.
I like to know that if I really need it fast, all I have to do is pull it out, no flicks, no opening, nothing that could possibly get messed up.
I also have a backup knife inside my PFD, that's a simple "hinged" one like Zanshin mentions, with a nub on the blade so that it can be swung open with one thumb. That one's much shorter, I couldn't keep anything more than two or three inches long in the PFD without a problem about 'folding' it.