The official GMAN Steam Community.

Garry's Mod Administrative Network or "GMan" for short is a full Garry's Mod addon manager. It grabs a list of downloads from a MySQL server and parses it into the application.

You can easily select an addon category, select an addon and click Install to install it directly into Garry's Mod, You can even Checkout SVN mods using GMan. Infact, you can even post your own mods!

GMAN Website[gman.org.uk]
Facepunch Thread[www.facepunch.com]