Get this and more (recently updated) FX plugins free from Destroy FX.
Bruce Sutherland JuceOPLVSTi is a VST version of the Yamaha YMF262 FM synthesis chip used to create the soundtracks of classic video games like DOOM.
Ton-Geraet 1 is a free plugin synth which is based on a SID chip and then taken further.
FamiStudio is a free simple, DAW-style, music editor for the Nintendo Entertainment System. It is designed for both chiptune enthusiasts and the NES homebrew community.
DefleMask is a free (Donations possible) cross-platform chiptune music tracker.
It is available for Windows, Mac OS X, Linux, iOS and Android.