I second the Earthworm Jim sentiment. The Sega CD version was my favorite as a kid. Sonic CD is also must-own.

Never played Popful Mail, but I've always heard good things.

You're all going to laugh, but I think Bram Stoker's Dracula's not that bad.

And I know this isn't technically a platformer (it's a beat-em-up) but Final Fight CD. I also like (another beat-em-up) Spider-Man vs. The Kingpin.