Megaman Mysteries covers the development aspect of Megaman, from old screenshots to unused sprites found in various roms. At first I was the only one doing this, doing my best to sift through the games and various sites to find what I could on this topic. Now, however, we have many others contributing to this effort such as people from Debug Modes and Unused Stuff or The Cutting Room Floor.
Where possible I've corrected past mistakes and replaced inferior sprite extractions with superior ones. However a lot of credit will go to these sites, which is why they are at the top.
Note long after I finished with this:
These pages contain a LOT of speculation and conjecture from a guy who knows very little about it and was just doing it all himself before any wikis or videos on the subjects came around. They're still up as an archive of the site's history, but there are better resources for this stuff these days.
If you like Megaman Mysteries,
you'll also like:
Games Tech (Japanese)
Bugman's Rockman 2 article (in Japanese)
Chris Covell's Megaman GB Proto review
Megaman Legends Station's MML Beta Demo
DASH!'s MM Legends Information Centre (archive)
Protodude's Rockman Corner