Cave Story is a freeware PC game that came out back in 2004, a 2D action platformer created by a one-man team named Pixel. Since then, it's come out on quite a few consoles (I've got it on my DSi, though it's on the Wii as well), and now it's available as Cave Story+ on the Mac App Store. The app is US$9.99 and available for download right now.
Wait a minute, you might be asking: Why do we have to pay $10 for a game you just called freeware? Good question. The original PC title is still downloadable for free, but the Mac App Version is a plus version with new graphics and music created for the WiiWare upgrade, as well as a new level called the Wind Fortress. Pixel is also reportedly planning to add more content to the Mac App Store version in the future, and this is such a lovely and awesome game that it's worth supporting with a few bucks anyway.
If you've never played Cave Story and you enjoy a good "Metroidvania" platformer, you're in for a treat this weekend. Go pick Cave Story+ up right now.