I know this show is pretty much old news, but I've recently started watching it on Cartoon Network. As with the new TMNT show, I hated it when it first came out (based on one episode that was heavy on the high-school drama). But the show is pretty darn wild, very true to the spirit of X-Men if not the details. (Most of the characters are in their teens.) And the more I see it, the more the cartoon from a decade ago looks dated and pedestrian.

So far in the few episodes I've seen, I've noticed the following characters:
Professor X
Wolverine
Beast
Storm
Cyclops
Rogue
Jean Grey
Shadowcat
Nightcrawler
Spyke
Iceman
Cannonball
Magma
Jubilee
Boom-Boom (ugh)
Multiple Man
Wolfsbane
Mystique
Toad
Blob
Richter
Quicksilver
Scarlett Witch
Magneto
Sabertooth
Colossus
Gambit
Pyro
Trask
Sentinel (there's only one so far, and it looks like it could take an army of the comic-book versions)
Angel
a British girl with purple hair I suspect may be Psylocke, despite a different first name
Nick Fury (!)
Captain America (!!!)

Airs at 10:30 Mon-Thurs on the Cartoon Network (just after Justice League, which you should be watching anyway). Highly recommended.