Alright, I think there probably was a thread about this a while ago, but I'll make a new one, since something this awesome deserves a thread at all times. It's somewhat old, but oh-so-worth it.
Frets on Fire
This is guitar hero. On your computer. You use the F1, F2, F3, F4, F5 keys as the guitar's frets and the enter as the pick button, just like you would in Guitar Hero. It's 33MB and it has high scores and the ability to edit songs and make your own. Contrary to Guitar Hero though, this lets you import songs, either ones you made or songs from a already growing database (link later). It's pretty low end, so pretty much any PC can play it. It comes with 3 songs, a somewhat easy one, a medium one and a hard one (each of these has a Easy, Hard and Amazing setting itself by the way). You hold the keyboard upside down, with the F buttons facing down. You can play without doing this, but it's tougher, less fun and generally fgty.
List of songs
By the way, do any of you know another database for this? With more songs? Have you made any?
Use this thread to post high scores and stuff too!
It's insanely fun, and like a modded Guitar Hero! Not that different from the real deal, so if you already own it you might want to skip this.