The japanese 8 bit band YMCK recently released their Magical 8bit Plug that creates some serious 8 but sounds. I am totally impressed, and am currently running my own copy on Digital Performer. The group states that the: "Magical 8bit Plug is a plug-in style software-synthesizer for producing primitive electronic sounds like the old 8bit game consoles. In Macintosh environment, it runs with AU (Audio Units)-supporting applications: ex. Logic, Digital Performer, Garageband. In Windows environment it runs with VSTi supporting applications: ex. Cubase. Most of the software/hardware synthesizers include the most primitive waveform: square wave and pulse wave. But almost none of them include the pseudo-triangle or the low-resolution noise that will most characterize the 8bit sound's unique taste. Magical 8bit Plug will help you producing the old 8bit sounds by providing those unique waveforms."
You can download the Windows and Mac OS X versions at the YMCK site: http://www.ymck.net/english/download/index.html