What (And Where) Are The Classic Free Games?

An anonymous reader asks: "I'm flying from the US to Europe in a few weeks and am taking my iBook along for the ride. With a seven hour flight looming, I'm wanting to take some games along to play. But I don't want to download a bunch of freeware that may or may not be junk; I just want the classics. What are the classic free games? I've already downloaded Nethack and am looking into Freeciv. What else is there? I need something that's multiplatform (or at least has a Mac port), something that's stable and has stood the test of time, and something that is more complex and engrossing than a Tetris clone. Thanks!"