Here's my build of the Devil's Triangle Drone Synth, designed by paulinthelab. Check out his site, he's got a ton of great circuits he's designed and posted for the general public to build, both audio related and otherwise.
This is a relatively simple drone box, with control over the frequencies of the oscillators, and a kill switch for each one. You can get harmonious sounds out of it, but I think it really excels when you have dissonance between the oscillators, or a bit of phasing. This is great as a base to layer further synth sounds over.