What is the best free way to convert a .wav file?

to an mp3 file? I downloaded a program to get excerpts of songs for a ringtone, but it will only put it in the .wav format and the only way to get the song as my ringtone is the get it as a mp3 before sending it to myself. I don't have any software for my phone so this is the only way I can do it. Please help?