If you are a musician, Tweakbench offers three awesome VST plugins for free: Peach (a NES-resembling synthesizer), Toad (NES drums), and Triforce (a synthesizer that closely emulates the sounds of the NES, plus an arpeggiator). Another very useful plugins is Bitboy by Darkware - which is also freeware, by the way. Finally, Magical 8bit Plug is not one to be missed, as it offers great quality, also demands no investment, and comes both in VST and AU forms.
If you do not own a sequencer with VST hosting capabilities, there are plenty of free options online. In despite of the probable initial learning curve, I trust the trouble of getting used to one will pay off when you start creating with those powerful tools. Of course, there is an infinity of different methods one could apply to make 8-bit music, but software emulation and simulation are the easiest and cheapest I am aware of.
Overall, it seems the scenario is getting better and better for those who, like me, had been missing the atmosphere that only carefully arranged bips and bops can provide. I truly hope you have enjoyed this first post of mine - hopefully, the first of many!
Thank you for reading, fell free to post your comments, and, please, do come back - as I've got a lot of nice subjects I plan to develop here.
* If you wish to listen to the soundtrack of The Immortal, you will have a much better time with the NES version, since the Genesis port featured less interesting arrangements - even though the themes are the same. There are other ports which I really never played - so, I really cannot testify on behalf or against their quality.