Activision has uploaded a demo of Guitar Hero 5 to Xbox Live. Assuming that's how it works. It's probably not.
Anyway, however it got there, it's 478MB and consists of three songs, which you can try out in the new jump-in Party mode.
Party mode allows you to pick up an instrument and join in with songs that are playing merrily along without anyone's help. The idea is that you can leave it on in the background during parties, obviously, and you can customise the playlist and everything.
It's one of many things we liked about the latest in Activision and Neversoft's venerable rhythm-action series, as you can read in our glowing Guitar Hero 5 review.