Can I use an Xbox controller with a Mac?
I've never used an Xbox controller with my Mac (I don't even own an Xbox), but TUAW says it works.
Absolutely. There is a driver downloadable free at https://github.com/360Controller/360Controller, an updated version of the Tattieboggle driver mentioned in a comment above.
It is the de-facto standard for XBox 360/XBox One controllers on OS X.
Installing it it in your system will add a new pane to the System Preferences, where you can check and configure your controllers.
To install it, download the latest disk image from https://github.com/360Controller/360Controller/releases and open the installer inside.