Maelstrom 3. Think Asteroids for the Mac. The old Mac. Mac OS. Pre-OS X. It's free, and lots and lots of Mac users wasted many a person-year playing it, but the thing is that it still works on OS X today. Maelstrom 3.0.6 fires up just fine on OS X 10.10.1 Yosemite in all its 640 x 480 glory, as noted by TMO member paikinho and shown in the accompanying screenshot. Note that it's not signed—if you have your System Preferences > Security & Privacy settings are set to allow apps downloaded from the "Mac App Store" or "Mac App Store and Identified Developers," Yosemite will squawk. You can launch it by changing those preferences (which I am not advising you to do), or by trying to launch it and then giving it individual permission in the Security and Privacy settings.
Check It Out: Maelstrom 3 Still Works on Yosemite, Still Free