I'm in the "no software" camp. It's been a long time, but I used to do it like this:
Play a chord or note on my Telecaster or 12-string; sample it into my TimeMachine delay, then slow it waaaay down to drop the pitch. Replay it into a D12 sampler, and sample it again, making a 5 or 6-second loop. Do this again, with a different note, and add a second loop in the D12, with a slightly different length. Play both loops at once - the different lengths ensure that they play out-of-sync - and play it through a reverb, then sample the result into my SP200, making a loop of around 30 seconds.
Repeat this procedure, with different sounds and reverb settings, until I have anywhere from 2 to 6 (or more) loops on the SP200 (each composed of several loops from the D12). Then, play all of them at once, adjusting the mix levels, until I get something I like. The odd loop lengths ensure that the whole mess stays very organic. By adjusting the reverb cutoff for the SP200 output, I can create a great drone that stays in a specific part of the frequency spectrum. After creating several loops like this (and recording them into my multitracker), I can fill the sound spectrum, without it getting too muddy.
By ensuring that nothing plays in sync (thus no MIDI), I can make drones that 'flicker' (sorry, that's the best way I can describe it), instead of just sounding like a brick on a keyboard.
Hmm, now you've got me thinking... maybe it's time to record something...