Ibanez SD-9 Sonic Distortion. Not made anymore, but it's an older pedal (80 somthing, I think). It looks like and electronically is the bigger brother of the Tube Screamer.
I've had this for a while, but only had an active 5 to try it out with. Needless to say, I was glad I got it free.
I sat down with my passive 4 string, and gave it a whirl. Pleasantly surprised. It does suck the low end a bit, but no worse than most bass pedals do.
It's got a pretty broad range of distortions, from mild overdrive to a raging fuzz type sound. However, the distorion is one dimensional, and a bit middy sounding, even with the tone all the way down. Tone all the way up, it sounds... er, different. This is a guitar pedal though, so I'm forgiving.
Using the amp, I beefed up the bass, and it rocked pretty decently. The middy sound is cutting and fairly distinct, though. This would work well in a rock band.
Pros:
Surprisingly little low end loss
Good range of sounds
Sturdy
Simple
Good for 4 strings
Cons:
Middy sounding
Does have low end loss
Not true bypass (but matching levels is pretty easy)
Doesn't sit well with active basses
Loses definition on the B string
Overall, I wouldn't have felt ripped off paying however much Boss charges for it's bass pedals, but I certainly am glad I got it for free. If you see one, give it a whirl, I think you might be surprised.