Rockman No Constancy - I have played that, it's probably the best MegaMan experience on the NES, the way the recreated the first level of MegaMan X is just awesome. Zelda Outlands is another nice romhack, so hard though. I did a few articles about those a while back. There's some killer Sonic hacks too, so professional!
Totally Rad is one I used to own when I was like 12...super good rock tunes on that one, insane bosses. Another weird one I happened on was Low G Man - imagine megaman or metroid, but you can jump about 4 screens high when fully powered up.
Thanks to all for your comments, I'm slowly working through the list. One I stumbled on that was pretty cool was SCAT Cybernetic warrior - kind of like contra/gradius. It has a pretty cool system that allows you to reposition your "options" in real time. Its WAY too hard though, I can't even pass level 1, it's the closest thing to bullet hell i've seen on the NES.
I tried Cocoran, no idea what was going on there!!