Genesis options:
TFM Music Maker
http://shiru.untergrund.net/software.shtml
I'm guessing that you're not too familiar with the concept of trackers, so be prepared for a steep learning curve. Sound wise, it's unmatched though, i think.
You can even find some presets ripped from Genesis games to get you started.
No sampling channel as of now though, so you can't reproduce the Sonic drum sound, for example. just pure synthesis.
VOPM
http://www.geocities.jp/sam_kb/VOPM/
a VST solution that is reasonably close. the interface is cryptic though. It doesn't model the Genesis soundchip, but one similar in architecture (bit more versatile)
If you're interested in the real deal, watch this blog:
http://little-scale.blogspot.com/
the guy has a midi interface for Genesis in the works.
8bit-wise, famitracker all the way.
http://famitracker.shoodot.net/
very good tracker, pretty much anything you could do on a NES is possible with it.
There are loads of VST options, and they're ok, you can make chippy sounding stuff with them alright, but if you want to make true chiptunes, the chip's limitations are an integral part of that, and famitracker gives you those.
If you don't care about polyphony limit or the dpcm and triangle channels fighting for volume (with true emulation come the same technical challenges that the composers had to battle against back in the day), here's a VST list:
Magical 8bit Plug
http://www.ymck.net/english/download/index.html
YMvst(Atari ST)
http://www.preromanbritain.com/ymvst/
Tweakbench (don't like these much personally but they're popular)
http://www.tweakbench.com/triforce
Basic 64
http://www.delamancha.co.uk/basic.htm
this one's cool. it's just c64 inspired, but sounds good and is fun to tweak.
I'll stop now ^^