This is not strictly a tracker compo, but it's very interesting nevertheless: KVR Audio hosts a monthly compo, the One Synth Challenge (https://sites.google.com/site/kvrosc/). Every month, a new synth (VSTi) is chosen and every participant has to make a tune with only this synth as a sound source (most free VST effects are allowed to modify the synth's output, though). This can be quite challenging and fun!