Frosts/Mora (laminated carbon steel version) - A $12 knife that has a blade as good as an $80 Benchmade? These really have to take first place, even over the beloved Opinel. Sure the handle looks kind of ghetto and the sheath is just enough to cover it, but heck it works.
Opinel No. 6 (carbon steel version) - For $7, there just isn't much at all that has carbon steel and a [mostly] flat grind (one side tends to curve a little). Weighs nothing, fits in your pocket, and is comfortable to hold. You can find more durable cheap knives, but not with the same devil of a razor thin blade blade.
Delica (FRN + VG10) - If you want a one-handed folder, you can't get a better blade for the price and on top of that it's a generally useful design too.