I've completed Blip_Buffer, a small library to handle the basics of band-limited synthesis, useful for writing a sound chip emulator. There's also a page describing why band-limiting is necessary and how it can be done for simple waveforms (it's how Blip_Buffer works internally). There are a few mp3 samples comparing audio quality and demonstrating the adjustable low-pass and high-pass filtering that's built-in. I'm hoping that this information or library will help with emulation sound quality, as virtually all emulators I've looked at use less-than-optimal methods.

http://www.slack.net/~ant/nes-emu/