The Boss LMB-3 isn't bad, and the BBE OPTO Stomp...both can be had for about $50 used.
Check out this website. The guy has reviewed about 100 different compressors, from pedal to rack-mount.
Here is a review of the Ashley model that Dave is referring to:
Ashly SC-50: This is a neat one- it's a one-rackspace line-level mono unit from the '70s, today much prized by studio engineers for use on electric guitar. Fortunately, it also sounds great on bass. It's a unique, rather colored sound- it has a slightly hollow quality with some grit and "wooly thickness" to the tone, sort of like an old tele P bass. This tonal quality is not as strong as an "effect", but rather a subtle coloration that gives a unique, bluesy, vintage character. The high frequencies are a tiny bit muddy and muted. It's not the most transparent comp, but the noise level is very low, the low end is not rolled off at all, and the compression action is smooth, with no swell of noise on the note tail. It has a decent range of usable and good-sounding settings, and it sustains nicely. It can be set up as a hard limiter, but like many comps it doesn't do as good a job of that as I'd like- it's much better at more moderate compression. At all settings there can be a bit of a "click" artifact when the threshold is hit. I really like this one, but it won't suit players who want a modern transparent sound. For what it's worth, the one I'm testing is the "blue face" model, the first version Ashly produced, which has slightly different components than the later "black face" model. The black one is supposed to be pretty similar though. Also, the in/out switch only turns the compression on and off, your signal still passes through the gain stages of the circuit when it's "off". If you're a blues-rock player, though, you won't switch it off!