MP3 Players:
Most mp3 players can now be set to output to a file instead of a sound device when playing. In this way they can be used to decode a mp3 to wav. Check the instructions for your player to find out how to do this. In WinAMP you set the output plugin to 'Disk Writer' and set the output path in [options]. You can decode a group of files by making a playlist. Be sure to set the output plugin back to waveOut or DirectSound when you are done.

Command Line Utility:
If you need a linear PCM file (WAV, AIF, SND or other) from your mp3's, it's easy to decode them with Fraunhofer IIS l3dec (which comes with the older versions of l3enc and the registered verison of mp3enc availble on the software page). This program is fully functional in the shareware release, all you need to do is enter the following at the command line and you'll have a huge file again.

l3dec filename.mp3 filename.wav -wav

You will recieve a message that says "EOF !" when the program is finished decoding, along with the number of frames and the length - this is not an error, just the message indicating that the program is done. You could also get a lost synchronization error during the last frame depending on the encoder that was originally used - the file should still be fine though.