TUAW reader Jon emailed in the other day to ask about possible game controllers for Mac, and while we've covered this issue before, it has definitely been a while. In short, the best way to use a game controller with your Mac is just to use one of the better controllers out there: the wired version of Microsoft's Xbox 360 controller (or, if you wish, any compatible third-party version). The port on it is just a standard USB, so it should plug right into your Mac. And in some cases that's all you need -- the OnLive app, for example, will make use of your controller right out of the box (which is actually easier than it works on Windows, incidentally, where you need to install an official driver), and there are other games around (I've been playing AirMech in my Chrome browser quite a bit lately) that will work great right away. I haven't tried Steam's Big Picture mode on my Mac yet, but I presume it too will work just as well.
If the game you're trying to play doesn't directly support the controller, however, then things get a little more complicated. There is a very nice driver for the Xbox 360 controller available for free online, and it will cover you for a lot of games that already support controller setups. But every once in a while you'll come across a game that requires you to use a keyboard, and for those times, you'll need to download a helper app, something like the Gamepad Companion (from the Mac App Store). That app will let you map the Xbox 360 controller's buttons to standard mouse and keyboard functions, so you can use your controller to do anything you want.
Apple seems closer than ever to actually bringing full apps to the Apple TV, and when it does, you have to think it will also create some kind of controller device (or include that functionality directly in iOS somehow). Until then, however, Microsoft's controller works perfectly well, and with a little bit of tweaking, you can get it to do whatever you need.