There is a great turnout for the Baltimore Wordpress Meetup Group at ADG Creative in Columbia, MD.
People are turning to Wordpress because it handles User Management out of the box.
example site: OnePhotoapp.com - Matt Taylor of OnePhoto gave a very informative presentation about how he built the platform using Wordpress.
TinThumb is an imaging processing plugin. There have been numerous recent security issues.
Post 2 Post allows you to create many-to-many relationships between posts.
WooCommerce (multiple payment processor options)
You can also use PayPal.
Stripe - Credit Card Payment Processor that integrates with WooCommerce.
Use Payment processors to handle credit cards. You do not want to be handling credit card numbers yourself. An exploit or breach can cost $250,000 in liability.
With Wordpress 3.x you can integrate in to the Admin Bar. This can clean up your site design.
Twitter's Bootstrap is a HTML5 complaint tool for UI development - Great for laying out a theme.
Artisteer is also a great development tool for layouts. There is a 30 day trial period for it.
Codex - The Wordpress documentation. Best place to go http://codex.wordpress.org
Next meetup will be on "You're not doing it right!"