As I continue to answer questions on the forums, I'm starting to notice that I reuse a lot of the same help and documentation links.  To make it easier for myself to find them (and for everyone else as well), I'm going to start compiling some of my favorites here:

Basics

http://docs.joomla.org/User_Group_Access_levels_explained_in_simple_terms

Security

Customization

http://docs.joomla.org/How_to_override_the_output_from_the_Joomla!_core

Development

http://docs.joomla.org/Naming_conventions

http://api.joomla.org/li_Joomla-Framework.html