Welcome to Geeklog Sunday, May 19 2013 @ 08:11 PM EDT
PortalParts.com is the homepage of (amongst other things) the File Management and Forum plugins.
gplugs.sf.net has a lot of plugins and other add-ons from various developers.
squatty.com also has a large collection of add-ons.
Geek Downloads has a lot of updated themes and other add-ons.