So, apparently NIcalis has added another platform to Cavestory's upcoming list. Steam.
If you haven't played Cave Story, play it. Now
www.cavestory.org
No excuse, just give it 30 minutes. If you like Metroid, you'll fall in love with this.
If you want to support the developer, you can buy it off of steam. It comes with a few extras. Time Attack, Achievements [Beating the hell level with only 3 max HP. Hah! Yeah right.], and the like.
Its an enjoyable game I've played through about 4 times. Give it a shot.