There's a FREE downloadable track pack for Rock Band 3 (for PS3 and Xbox 360 platforms).

Look for the "Rock Band Free 01" pack when you're in the Xbox Marketplace or Playstation Store. Not the most popular artists, but it's 9 free songs. Below are the included songs/artists.

"Visions" Abnormality
"Get Clean" Anarchy Club
"Night Lies" Bang Camaro
"Shoulder to the Plow" Breaking Wheel
"Welcome to the Neighborhood" Libyans
"A Jagged Gorgeous Winter" The Main Drag
"Conventional Lover" Speck
"Supreme Girl" The Sterns
"Rob the Prez-O-Dent" That Handsome Devil