While MP3s are currently surrounded by controversy, they are in no way illegal. In the same way you can make a tape of a CD, you can make unlimited MP3 copies of any CD you own, for your own use. 
Distributing MP3s, though, is illegal (unless you own the copyright,
 or the songs are in the public domain). So before you e-mail MP3s to friends or post them on a web site, make sure you are doing so legally.

Many of the MP3s offered on file-sharing programs such as Napster and Gnutella, however, are illegal, offered without the consent of the band or its label. While no individuals have been prosecuted for sharing MP3s as of yet, many users distributing MP3s on Napster have been kicked off the service for sharing Metallica files. (
The Record Industry Association of America maintains that if a computer is set up to behave like a file-sharing application, and that computer makes copyrighted material widely available, the user is in violation of the Digital Millennium Copyright Act.) 

There are many free legal MP3 songs (with copyright holders' permissions) on the Internet, some of the finest are available for download free at the Music Downloads section.