Below are just a few of the resources, outside of WordPress.org, that I use to maintain my 20+ WordPress web sites.

Simple widgets. Widgets 101.
http://wpbits.wordpress.com/2007/06/16/simple-widgets-widgets-101/

WordPress Plugin/ Template list
http://mashable.com/2007/08/16/wordpress-god300-tools-for-running-your-wordpress-blog/

Top 10 Tricks to Master Your WordPress Blog Empire!
http://top10tricks.com/report/wordpress-setup/

How to Easily Manage Multiple WordPress Sites With One Database and One Code Base
http://sitening.com/blog/how-to-easily-manage-multiple-wordpress-sites-with-one-database-and-one-code-base/

WordPress MU, or multi-user, is designed to do exactly that. It is most famously used for WordPress.com where it serves tens of millions of hits on hundreds of thousands of blogs each day.
http://mu.wordpress.org

30 Excellent WordPress Video Tutorials
http://sixrevisions.com/wordpress/30-excellent-wordpress-video-tutorials/