Hey all.
Thought I'd offer a quick plug for an excellent new(ish) site dedicated to Sega's MegaDrive-related formats (MD, Genesis, Mega/SegaCD, 32X, etc) that I've been helping out with: http://www.guardiana.net/
It has evolved out of a French site originally dedicated to Landstalker and other 16-bit RPGs, but now features the most detailed database I've seen for all MD/Genesis-related games (currently just over 1200 seperate titles), with cover pics and details on releases from all regions (Japan, US, Europe, Brazil, South Korea, Asia, Canada, Australia + more) aswell as reissues/special editions/etc.
Integrated into the database is a nice collection listing feature (as an example, you can see mine here) which also offers a neat visual representation of your entire collection by clicking on the little [+] button at the top of the page.
The bulk of the site is still in French, but an English version is currently in the works. I've been getting along fine so far with a combination of high-school French and Google Language Tools German, Portugese and possibly Japanese versions are all also planned.
Also in the planning stages is a complete hardware database to complement the existing software listing.
The forums (http://forums.guardiana.net/index.php) offer the opportunity for discussion of all Sega 16-bit stuff, and there's an English-language sub-forum that has just been opened (here). Several of the French members can also speak good English, so don't be afraid to post elsewhere either...
While the database is pretty detailed already, it's still growing on a daily basis too, and any information on missing games, variations, cover scans etc are all very much welcomed - the website staff are very good at updating it, and new information is usually added within 24 hours.
Most of the members so far are based in Europe, but we're starting to expand beyond that now. I don't think there are any US-based members yet though, so come along, have a look, and join in & help us out if you can!