i tunes does it.
go to edit > preferencesthen click the advanced tab then importing.
select mp3 and the bit rate you want to use... 192 is standard but you can have higher or lower quality (although they'll take longer to convert)
then go back into your library and select all the tracks you want to convert and right click... there should be an option to convert to mp3... click it and it will copy all the highlighted songs... then you can delete the old files (if you aren't sure which are which select 'kind' on the view options...)