It's a megamix of various stuff from the network. Here's what it is.
Based on the title, Megamix 01, it's easy to guess there will be more of these. It seems like a good way to show off and market some of the stuff on the service. Here is a link to download it for the Xbox 360*.
The songs featured in the sampler are:
Rose Of Jericho – “Buried Cold”
James William Roy – “Paper Valentines”
Glass Hammer – “Sleep On”
Robby Suave - “Everyone I Know Is An Alcoholic”
Scratching The Itch – “End Of This”
You Shriek – “No Heroes”
Voodoo Pharmacology – “Minnesota”
Chrome Coma – “Bow Down”
Freen In Green – “Frontier Factory”
Paul Henry Smith & The Fauxharmonic Orchestra – “Night On Bald Mountain”
Counterfeit Pennies – “Five More Minutes”
Single White Infidel – “Inheritance”
You can find more info at http://rbnmegamix.com/
And if you don't already own them, here are the previously released freebies for Rock Band, also for the 360*:
* Sorry that all links are XBLM. I don't own a PS3 and I'm not sure how the storefront works on that side of things.