I've posted about good ol' FamiTracker
in the past. Great for making spot-on NES tunes. I've taken a bit of an interest over the last year or two in down-sizing tunes that would actually play on a real NES (not those poopy MIDI conversions), and this evening, I did a cover of Phantasy Star IV's title screen theme off the cuff since it's one of my favorite game tunes.
You can listen to said tune here:
You can have a look at the FamiTracker Module here: (you need FamiTracker to open it)
For this cover I made use of Konami's VRC6 expansion.
Since I tend to do all my cover tunes off-the-cuff, there's a bit of room for error, but I was really happy with how this one turned out. Has quite a bit more ass-end to it than the original Genesis/Mega Drive version, and the NES' square wave outputs aren't as "twangy" as the FM outputs of the Genesis.
I'm considering doing a few more Phantasy Star IV (or others in the series) in the future since this one turned out so well.
Hope you enjoy!