I just downloaded this game, it's basically guitar hero for PC, Mac, or Linux. Rather than using a guitar controller you hold the keyboard like a guitar and use the F1 - F5 keys as frets and the enter key to strum. It sounds awkward but it works really well. You can download it free from:
this is just the basic version and it only has 3 songs but the best part is, you can download new songs for free. If you like a song there's a good chance someone else has made it for the game. Here's a site with a couple of song packs from the guitar hero games:
It's also worth noting that you'll need a somewhat decent PC to run this, preferably made in the last century. It's a super fun game, I recommend everyone check it out.