Joomlart
Joomla template site that offers both a template club and an extension club. They also have a pretty decent tutorials section.

RocketTheme
Another theme site that provides some great extensions, free and premium Joomla templates. Rockettheme does monthly releases typically on the first of each month.

Gavick
Joomla template club featuring a number of unique templates. Also a few extensions available.

Joomlashack
Joomlashack is one of the most popular theme sites that provides  free and premium themes for various versions of Joomla. They also have a number of extensions and learning resources available.

YOOtheme
YOOtheme is a premium theme provider, they offer a number of themes with monthly releases as well as some other tools such as Zoo (a catalog system for Joomla) and some extensions.

A simple tip from
Ultimatecine.com