A site we've recently discovered, and instantly enjoyed, is Strobist. As the name suggests, it's a site all about lighting, by Baltimore Sun photojournalist David Hobby. Strobist has a simple goal: to help photographers make better use of shoe-mount flash units.
With a light-hearted writing style, Hobby tackles the basics in Lighting 101 and covers more advanced topics in On Assignment (which includes breakdowns of how he lit various jobs for the paper). Strobist is definitely worth a look.
Thanks to Doug Urner for bring Strobist to our attention.