Games that are native to some version of Windows but can be played on *nix operating systems such as OS X or Linux using WINE (Wine Is Not an Emulator).

A re-creation of the original Half-Life in the 2007 version of the Source engine, with upgraded high definition audio and video-- up until the Xen chapter, at least. There's no native OSX version, but if you install Steam into a WINE wrapper, you can use it to install the 2007 Source SDK and play it on OSX.