[Coda]

X11 is bundled with Leopard and Snow Leopard, so you can build most Linux programs for OSX. There are a couple of projects that automate the dirty work for you -- Fink and MacPorts. (Don't use both at the same time.) They're just normal package managers except they compile the stuff you select for your computer's configuration.

I'm surprised you haven't heard of WINE if you're a comfortable Linux user. It's a compatibility layer that provides equivalents of a lot of the Windows operating system so you can run many Windows .exe files under X11. Compatibility isn't perfect (it's pretty hit-and-miss) but it works pretty well.