A group of independent game developers have released a "Humble Indie Bundle," and if you're a Mac gamer, it's a treasure trove. For a donation of your choice (the retail value is set at $80, but you can pay what you want, and you can even specify whether your money goes to the game developers or a couple of different charities), you can pick up the indie classics World of Goo, Aquaria, Gish, Lugaru, and Penumbra Overture, all playable on Mac, Linux, and Windows.
Amazing deal, and this is about the most generous and customer-friendly way to do it. Any one of these games is well worth the purchase, and not only are you supporting developers and charities, but you're supporting the decision to release great games like this cross-platform, without DRM, and without a publisher or middleman getting in the way and taking a cut. Excellent deal -- they've already raised over $31,000, and if you play games on your Mac at all, definitely head over there and put ten or twenty dollars in the pot to support the bundle.