last.fm - Large catalog of music that includes free mp3 downloads from popular artists.

Accuradio - Has a selection of formats. I like the 60's selection. Ad supported, so you have to click on an ad to activate

99x.com - Alternative rock. Uses a popup player so adjust your popup blocker accordingly

The Agriculture - Unusual and definitely Alternative music

KEXP 90.3 - Great online radio station. Located in Seattle, it provides a variety of music; Alternative, Electronic, Jazz, Hip-Hop.

Go Home Productions - Good site for Mash Up music downloads