IMFavorites - A useful tool for filling CDs/DVDs/Mp3 players with what IMMS knows you like.
IMMSView - An IMMS-aware playlist replacement for XMMS written in Python.
mmsv2 - Mpeg Menu System V2
immsscript - bash script for querying imms' database to create playlists and adjust ratings (multiple files at a time)
export to iTunes - A script to export IMMS-learned ratings to iTunes.
If any of these have features that you wish were in IMMS, email the Mailing List to see if they can be added.
Dynamic Taste Detection - An XMMS patch for song rating.
gjay - Perform background analysis on your music to generate smooth playlists.
Mirage - acoustic analysis library and plugin for Banshee.
autoqueue - a plugin for Rhythmbox and Quod Libet using Mirage.