Megaman Unlimited
http://megaphilx.com/?page_id=26A Mega Man fan game that is insanely difficult, even by Mega Man standards. Seriously, I've logged like six or seven hours on this game, and I haven't even beaten one boss!
I think part of what makes it so hard is the marathon-length levels; it's a challenge just to get to the checkpoint on many of them! It's got most of the traditional Mega Man staples, like E-tanks, an item shop, and the slide, but no charge shot. (I actually like that it doesn't have the charge shot, because all the charge shot does is waste time, since you're almost forced to compulsively stop and charge for a moment every time you shoot for optimal gameplay, unless there are more enemies onscreen to distract you!) The music, as is typical for Mega Man games, is quite good. Another thing that makes it difficult is that there are a LOT of spikes. And the floating platforms in Trinitro Man's stage will drive you insane until you figure out how to get past them, and even then some afterward as it's tricky to pull off, and the spike guard won't save you from the lava/napalm/nitroglycerine/whatever that glowing liquid stuff is. I find it easier to play this game with a gamepad than with a keyboard, but one problem I had with my Logitech gamepad is that the diagonals don't seem to register. This isn't a problem most of the time, seeing as Mega Man doesn't use diagonal movement very often, but it can be a problem when controlling Beat after he rescues you from a pit. If you don't realize that you can't move diagonal with the gamepad, you're likely to fall back in as Beat runs out of time!