My top picks so far:
• If you need something cheap, the best pedals in my opinion are the DOD Milk Box, Boss LMB-3, and Line 6 Constrictor, pretty much in that order. Also the first edition of the BBE Opto Stomp was really good (don't buy the mustache one). If you want a rackmount unit, get the Alesis 3632. Others that are pretty good for the price include the Biyang CO-8, Moen Uni-Comp, Alesis Nanocomp and Microlimiter, Ibanez BP5, Guyatone ST2, dbx 163X, Behringer DC9, and ART Levelar. And if you want a DIY project, the BYOC Optical kit is a very good value.
• If you want something a little better than those, but still not expensive, the Pigtronix Philosopher's Rock and Phil. Bass, Seymour Duncan Double Back, and Carl Martin Classic Opto all provide a lot of bang for your money.
• The top tier pedals that I'd recommend to most people combine ease of use with great tone and low noise: the Whirlwind OC Bass, Diamond BCP-1, MXR M87, 3Leaf PWNZOR, FEA SMX, Empress, Maxon CP9Pro+, Rothwell Love Squeeze, Effectrode PC-2A, Mad Prof Forest Green, Dedalo Gorila, G-Lab BC-1, Wampler Ego, Aguilar TLC, and Earthquaker The Warden. If your tastes are more exotic or particular, the Markbass Compressore, Durham Sex Drive, Carl Martin Comp-Limiter, OKKO Coca, Tortenmann TK-23, FEA's more knob-laden models, Taurus Tux, Joemeek FloorQ, Origin Cali76, and Menatone J.A.C are also very much worth your attention.
• For rackmount units the Rane DC24, dbx 160X/XT, Ashly CLX/DPX series, Symetrix 501, Aphex 661/651, and JBL 7110 are all excellent. They each have unique qualities that make it hard to pick one "best" one. On a tight bugdet, the new Alesis 3632 and the old dbx 166 are very good values. I am also very fond of the FMR PBC-6A, ADK CLA-1, and Joemeek MC-2, but they are half-rack modules, sometimes difficult to mount in a regular rack rig.
• Tube compressors: my favorites are the Effectrode PC-2A, Retrospec Squeezebox, and Markbass Compressore, though the Lightning Boy is worth your time too. Read this article for more info about tube comps.
• What do I use personally? Lately it's the Origin Cali76, for its organic tone and action. The FEA's (all of them) are also close at hand. Other personal favorites include the Empress, Markbass Compressore, G-Lab BC-1, Diamond BCP-1, Aphex 651, Retrospec Squeezebox, Frantone Sandwich, Durham Sex Drive, ADK CLA-1, and Effectrode PC-2A.
• Limiters: the Rane DC24, Valley People Dynamite, Aphex 661/651, MXR M87 Bass Comp, Maxon CP9 Pro+, Pigtronix Philosopher series, Alesis MicroLimiter, Guyatone BL2, Boss LMB-3, various Joemeek models, Markbass, Demeter Compulator, FEA Comp-Limiter and DE-CL, Menatone J.A.C, Ashly DPX/CLX/CL series, Aguilar TLC, and Symetrix 501 all do a pretty good job of cleanly limiting big signal peaks.