www.gavtrain.com

Gavin Hoey provides a fantastic blog complete with videos covering all sorts of photographic topics both on location and in the studio finishing off with some great Photoshop tutorials.

www.strobist.com

All things related to lighting with off camera flashes using equipment to suit all budgets.  A vast resource of posts showing behind the scenes shots and background information.

www.ephotozine.tv

An archive from a whole range of contributors (including Gavin above) covering all photographic topics including technical advice and equipment reviews.

www.planetneil.com

A superb website covering a wide range of portrait and wedding shoots using on and off camera flash. 

www.youtube.com

The worlds largest store of online video clips.  Look in particular for Mark Wallace of Adorama TV for easy to follow how-to clips.

www.sylarena.com

As well as a great portfolio, Syl publishes two blogs with great information on flash techniques and general photography.