The Boss GL-100 is highly prized analogue rackmount overdrive and distortion unit it has a quite a few different modes, from subtle overdrive to full on distortion, its like the best of all their distortion pedals built into a rack unit, its got 2 channels, channel 1 has 5 different overdrive and distortion settings and a 4 band EQ, The second channel has 4 settings, two distorted and a 3 band EQ and it has balanced +4 output.
Actually i am selling mine at the moment for £100 if anyone wants one....
Read up on them- great units.
actually it's not rackmountable (at least not mine)
i'm not a massive fan of the super distorted sound on 303's but If i do want some I use either the audio in part on my korg es-1, overdrive my mixer's input, or crank it through my midiverb 2, I've yet to find a pedal that gets it right...
Distortion in like anything else, you have to find the one you like. Classics would be the Tubescreamer, Pro-co Rat, MXR and Big Muff. I had a really cheap multi pedalboard once that had a flippin awesome distortion, but I can't remember what it was. Go to a guitar shop and demo some but stick with analog imo. Grab a cheap behringer clone of a Tubescreamer, you can get a few with that route.
Or the input gain on the channel of the right desk.
I know that a lot of you guys would kill me for talking about "virtual stuff" now and not using the real hardware thingy but I achieved the most interesting distortion for a 303 (that came pretty close to the old chicago records from my collection ) with a selfosc 4 pole lp ladder filter where the drive is placed at the filter output but before the feedback loop. If I remember correctly I used maximum drive, really low cut off and pretty high resonance settings (and full kbd). After that I controlled the amount of distortion with the envelope amt knob while I finetuned the filter envelope. Very distinctive and deep distortion. Perhaps you could achieve similar results with some real hardware.