I use VLC myself. Small, light weight, plays any audio or video file you can possibly throw at it and it works on Mac OS X, Linux and Windows.
If that doesn't float your boat then foobar2000 is another brilliant option.
One question I do have though is why you are still using MP3? I can understand it for tracks you already have but if you are ripping CDs and you want a lossy codec then Ogg Vorbis or AAC offer much better sound quality for the same bit rate. Of course you could go even better than that and use a lossless codec like FLAC which is identical in quality to the CD you are ripping.
Edit: One important point though is that you can't convert from MP3 to AAC or Ogg Vorbis (well technically you can but you'll lose even more sound quality). You should re-rip your CDs if you want to change to those. Or better yet rip the CDs to FLAC and then keep the FLAC files and make a copy in Ogg Vorbis or AAC for your mobile phone.