Here are some folder suggestions based on my experiences and what others here say...approximate prices too.
0-30$
8~30$+ - Various Swiss Army Knives. Some of my favorite models are Ramblers, Recruits, Soldiers, Farmers, Tinkers, Bantams and Nylon Solos
6~20$ - Opinel Knives. Great traditional feel, easily customized and some of the best slicers at any price point.
30$ - Buck 110 / 112. Probably the stoutest knife you can get for 30$.
15~25$ - Spyderco's Byrd line.
18$ - Kershaw Vapors. Just under 20$ for a compact little framelock.
20$ - Kabar Dozier. Various blade shapes in a solid lockback.
31-75$
40~50$ - Spyderco Native. Superior ergonomics to the Delicas IMO, S30V steel well thought of here and easy to find for 40$ at Wal Marts. Blade stock is a little too thick to me though.
40$ - Delica / Endura. Same knife, different sizes. Great quality and design for the money, but the ergonomics just didn't fit my hand the way I'd like them too. Well designed FRN grip patterns, steel liners, great blade steel and overall solid contrsuction.
50~60$ - D'allara Drop point. A step above the Endura. Beefier, more comfortable handle. Ball lock. Wire clip. Good stuff.
40~60$ - Kershaw Leek. Assisted Opener, compact size and a nice wharncliffe profile. Lots of color and handle options available.
50$ - Buck/Strider 889. Beefy, solid design based on high-end Strider designs.
45~60$ - Benchmade Griptilian. I prefer the D'allara, but many really like these knives. A little wide, but they have the much loved Axis lock and pretty comfortable handles. The only thing they have on the Spyderco to me is an easier to manipulate lock release.
30~50$ - CRKT M18, M20's etc. Nice linerlocks with LAWKS or AUTOLAWKS, amazing fit and finish for the price point and pretty comfortable handles. Flippers too, which make them very fast to open.
30$ - Kershaw Storm. Solid framelocks with good blade steel for a low price.
76-150$
100~135$ - Spyderco Manix, Mini-Manix, Military and Paramilitary
120$ - Benchmade 710
That touches on some of th ebasics, should give you an idea of what to look for.