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.
3 free game console synthesis plugins. Emulations of some of the most iconic chips including the Ricoh A203 (Nintendo), PAPU (Gameboy), MOS Technology SID (Commodore 64, 128 & MAX) and TI SN76489 (Sega, BBC etc).