Of the game controllers available for home computers, only a few are officially supported on the Mac, but you can often get others, like those for the PS3, Wii, or Xbox, working just fine.
While the Mac's keyboard and mouse ought to be enough for most purposes, having a dedicated gaming controller can give you the feel of playing with a gaming console. If you own a Mac and have a few games you would like to try, then you might also be interested in getting a gaming controller or two up and running with your system. While the Mac platform has not been noted for catering to hard-core gamers, there are still many popular gaming titles and gaming accessories available for the Mac OS.