http://agtp.romhack.net/images/projects/cavestory/0.png
I got this game as part of The Humble Bundle 4. In fact, I got the bundle just for Cave Story and Gratuitous Space Battles. If you loved old NES or SNES side scrollers you need to check out Cave Story. Playing through it with a USB SNES style controller is the way to go.

I technically have Cave Story+ but turned the settings to only show me the old graphics, those seem better to my eye.

I haven't beat it yet, just put a few hours into it here and there. So far, really really fun.