Jamlegend

FretsOnFire has been around for a while and allows you to play GuitarHero on your own machine, but JamLegend is an online version. Using the 1-5 buttons for the frets and enter to strum you can play like Guitar Hero.
There is some pretty good music on there from bands you will have never heard of - I didn't see any music that I recognised, but I guess this will eventually come, but in the meantime this is a fun way of finding out new music. One of my favourite tracks so far is Go On by Blue Judy and the neat thing is they also provide links to other sites where you can get the music to download. In this case there is itunes, cdbaby, lastfm and others. The other good thing is they have good taste in tshirts - see the right most tshirt ;-)
There are showdowns and Duels where you can compete against other people in realtime. It also has social features where you can add friends and you need an invite to play - I currently have three invites for (regular) blog readers if anyone wants it leave a comment.
One down side that is really annoying is that my flash player stutters - normally when I get to around 50 notes which means it breaks my streak and I have to start again. Seeing as though continual play is the best way of advancing through the ranks this is REALLY annoying. I'm not sure if it's a problem with flash - I suspect it is as it also affects youtube and other flash video viewers :-(