xmms-shn is an input plugin to play shorten (.shn) files in XMMS. It supports real-time seeking via the seek extensions built into shorten 3.x. Shorten is an audio compression format developed by SoftSound, and is used extensively in the distribution of losslessly-compressed live music.
xmmstcl lets you control XMMS playback, skip, pause, change the playlist, change skins, and do everything else provided by the xmms_remote_* methods, which is almost everything from the standard user interface. It also has some things not available there, such as "XMMS onchange" which lets your Tcl/Tk application know when something happens in the player. The companion "TkPlayer" has samples for the most important functions and will keep XMMS busy by generating random playlists with songs that you haven't heard in a while.