General

The Last.FM Scrobbler lets you use the last.fm service to find people with similar musical tests and get personalized music recommendations.

While not strictly a plug-in, Evil Lyrics is a great tool for tracking down Lyrics and seeing them in sync with your music. Just install it, and run--the lyrics for whatever track is currently playing will appear on-screen.

Minilyrics is a great tool for looking up and displaying lyrics within MediaMonkey. What's really nice is that it can be fully embedded within MediaMonkey. Install by closing MediaMonkey, running the installer, and then running the Embedded Minilyrics plugin. When you run MediaMonkey click View > Minilyrics to cause the lyrics dialog to show up.

Subsonic isn't a MediaMonkey plug-in at all, but it can augment MediaMonkey by giving you remote access to your files via a web browser. See the author's site for installation instructions.

Wawi is a web interface plugin originally written for Winamp. It lets you control the MediaMonkey player and the Now Playing list, but doesn't give you complete access to the library. It's a great solution if you need limited remote control of MediaMonkey via a web browser. Install to the MediaMonkey directory and then run MediaMonkey. The readme includes additional configuration information.