JamLegend, to put it simply is Guitar Hero in a browser. As long as you have access to the internet and have a keyboard, you can play JamLegend and it is free. Playing is very simple. Once the song starts notes come down as dots on a Guitar Fret and you have to hit the correct number keys and hit enter as the notes pass by.
If you have a separate keyboard you can also play "Jam" style by holding your keyboard like a guitar. If you think you look ridiculous holding a keyboard like a guitar, some people have reported success in using plastic guitars.
You can also have one-on-one duels with your friend or a showdown which is a multiplayer version. You will be competing in real time with other players. jamLegend is in beta and the company has plans to introduce facebook and myspace applications. Right now songs are available from about 50 Indie artists who have agreed to make their songs available on the site. But eventually, the company plans to introduce commercial songs which might be a pid upgrade for users.
The application is well made and if you are into Guitar Hero style of games, it can be quite addictive to play against hundreds of other users. Except for some minor freezes, I did not encounter any serious bugs in the application. Since JamLegend is out of the invite only stage and in public beta, there is no reason no to try it. Rock On!