Major Nelson has revealed that two new demos are available on Xbox Live Marketplace. The first is Guitar Hero: Van Halen. It's like Guitar Hero, but with Van Halen in it. At this point, if you need more description than that we can't really help you. The demo is available in all regions.
The other demo is for Crash Time 3. Honestly, how that game ever made it past Crash Time 1 is a mystery we will never solve. The demo is available in European regions only. You can add either demo to your 360 download queue by clicking the links below.
Shortcut: Download the Guitar Hero: Van Helen demo [Via Xbox.com]
Shortcut: Download the Crash Time 3 demo (Europe only) [Via Xbox.com]