Jeff Minter*'s [link|http://www.versiontracker.com/dyn/moreinfo/macosx/25447|GridRunner++]. All hail Yak. I played it on the Vic20 and the C64, and here it is once more, with feeling. And sheep.
Great fun, especially the turbonutterbastard mode, or whatever it's called...
* All hail...