Not sure of specific tools as such, but I believe the issue with seamless looping of MP3 is mainly due to padding silence to frames, where a frame is a fixed number of samples (1152 acording to the page below ) and a few other idiosyncrasies.
This Make Gapless MP3 page should help. The important information is at the bottom of the page (Theory of operation), though I noticed some programs are mentioned at the start.
