The newly reopened Descent Mission Database, that contains many singleplayer and multiplayer user-created maps, can be found here:
[url=]http://www.enspiar.com/dmdb/[/url]
Feel free to sticky this.

Old site died few months ago but good people @ descentbb.net have restored it.
New link: <span class="bold">Descent Mission Archive</span>

Excellent - Sticky this!!