http://fretsonfire.sourceforge.net/

This one is for all the Guitar Hero fans out there (which is a lot, considering the franchise has made over $1 billion in North America alone). Frets on Fire is a free, open-source game for the PC inspired by Guitar Hero, the gameplay involves emulating playing a guitar by hitting the right notes as you strum along, getting the right rhythm and timing is key. To play Frets on Fire, you can use your keyboard instead of a Guitar Hero controller by flipping it around and using the F1-F4 keys to hit the notes. However, you can use the official Guitar Hero controllers too! Some will plug straight in via USB while others (like the Wii version) might require a bit of tweaking but will work wirelessly and give you the feeling of playing the regular Guitar Hero with all the benefits of Frets on Fire!

What are the benefits exactly? Well apart from being totally free, there is a dedicated community supporting Frets on Fire, creating mods, skins and (best of all) tons of custom songs to play! Not only can you find the entire Guitar Hero back catalog of songs but plenty of other tracks by different artists. You can even make your own songs to play with the built-in song editor. A great place to start finding custom songs is the Frets on Fire wiki and the Frets on Fire fan forum where you can find hundreds of songs, including different difficult levels to suit new players all the way up to rock legends.