Marvel UA2 demo on Xbox Live
Thor you might like a try.
Activision has released a Marvel Ultimate Alliance 2 demo on Xbox Live.
The full game's out but not brilliant, which you'll know if you read our MUA2 review. Perfect demo fodder, then, and not too large at 706MB.
Unfortunately there's no accurate description of what the sampler offers. An enormously inaccurate description would be something like "you fight some cats using bleach". Er, expect an early level and small portion of the superhero roster to beat up the baddies with.