mp3 surgeon is not free, look on google. He also has ogg surgeon which
allows you to join ogg files. Ogg files can be concatenated however some
programs can't handle multiple streams in ogg files.
If you are joining mp3 files together for a podcast, chances are that you want to edit the files for continuity. There is nothing wrong with this as long as you are aware that every time you reencode an mp3 file there will be some loss in quality. The loss may be so slight that noone will know but it is not the same as joining the mp3 files with a program like mp3 surgeon. There may be a free mp3 joining program out there, who knows.