Did you know that Last.fm has a free mp3 download section on their music portal ? The Weekly Free Downloads category of Last.fm lists currently 189 songs that can be downloaded to your computer without charge. All songs are available on one page sorted by popularity. You do find some popular artists among them including Snoop Dog, Sarah Connor as well as some classics from Beethoven and Bach.
A download starts when you click on the Free link next to the artists name and song title. It is possible to listen to the songs before you start the download by clicking on the mp3 link in that row. The songs can be downloaded by visitors and users alike, no registration is needed for this.
One thing that bothers me a little bit is that the songs cannot be sorted by genre. I did however find a link on Liquid Parallax that pointed to a website where Last.fm users where categorizing the free mp3 downloads. Just click on one of the genres in the tagcloud which will load a new site where all artists of that genre that offer free mp3 downloads are listed.