Via Adrian Anders and the CDM forums, where we’re trying to figure out why it’s crashing someone’s installation of Ableton Live. Totally free, totally cross-platform (Windows VST, Mac AU). But for the next best thing, and the ability to do evil things to those sounds within a serious host, this free plug-in from Japan that will do the job:
The best way to get 8-bit sounds is to employ a real NES system or Game Boy for fifteen bucks and go to town - nothing else sounds quite the same.
