There is one I found called "8-bit Plug" that is a free VST you can download.

"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."

Here is the website: http://www.ymck.net/english/download/index.html