A simple volume monitor. Unlike the builtin xmobar one, it doesn't poll but only updates when informed by alsa of changes.
A battery monitoring script. It's a wrapper around upower (so you'll need that installed). Unlike the builtin xmobar monitors, not only does it work consistently, but it also doesn't poll.
This is a script that I use with xmobar to control mpd. It displays current track information in a pretty form for xmobar as its primary function.
As a secondary function, whenever the play queue is empty, it enqueues a random album (random album by random artist) to the queue. This is great for people who work well with music playing, but for whom trying to find the next album from their library to play is time-consuming (and also for people who like to listen to full albums).
It should probably be noted that my xmonad config has keys bound to clearing the current play queue.