Marathon, developed by Bungie in 1994, is a pretty cool game. I've just now started playing it on my MacBook and it is pretty addicting. The nice thing about Marathon is that it actually runs really well on my MacBook (2Ghz CoreDuo, 2GB RAM, Intel GMA 950). The other great thing about Marathon is that it's open source and is available for OS X, Linux, and Windows. If you have some time to kill and want to get your hands on Marathon, it can be found at source.bungie.org. If you download Marathon, don't forget to grab the HiRes textures to enhance your experience, they are worth the 222mb download.