I did a couple of quick mods on my MXR M-86 Classic Distortion Pedal. The thing is sort of flat sounding so I did a quick google search and found a couple sites with some good info on the pedal in question. The most poular mod is to remove C4, and that made the pedal a bit too bright, almost noisy. So I kept reading and one of the members over at DIYstompboxes.com had recomended using nothing smaller than a .01uf and that seems to really brighten up the sound. I attached the old cap and the new cap to opposite ends of a SPST switch and now I can switch back and forth. I also added a Blue LED, it does not really effect the sound, but is much cooler than standard red.
All the info can be found by following these links.
FreeStompBoxes.org has a bunch of pages of dedicated to these mods in the forum, so check that link for all the juicy details. Photos of someones mods can also be had at DIYstompboxes.com