If you'd like to play something like Guitar Hero on the PC and for free take a look at Frets on Fire.It's a Guitar Hero clone and the songs are created by the users (yes, you can find all the songs of the original Guitar Hero titles all over the place) and it's highly customizable. There are even MacOS X and Linux versions and a version you don't even need to install.
If you think it is not possible to enjoy the full awesomeness of Guitar Hero without having the guitar controller, then look at this!
...and again.
....and then you notice it's 4:00 AM and you need to get up early on the next day.
(not that it happened to be of course!)