1. Runner's World Online - For those of you familar with RW magazine, this is definitely not the magazine; it's far, far better. The site is updated daily with current information on running and racing.  The site also has an externsive archive of tips and training programs.  It's worth checking out on a daily basis. 

2.  SMI - Performance, Prevention, Rehabilitation  - Strengthening, stretching, and injury prevention, the SMI site shows you how.  This is one of the few sites on the web which has clear, step-by-step pictures which demonstrate different exercises.  Even if you're not hurt, this is a good site to visit periodically to remind you what you could be doing to stay off the injury reserve list. 

3. Dr Pribut's Running Injuries Page - Ok, if you haven't done all the good stuff that the SMI people have suggested, then you've likely developed a running injury.  Here's a monster collection of advice on how to heal it. 

4. Run Down - This is the Yahoo of all running sites.  It's a very well organized and comprehensive directory of almost anything you'd want to know about running. 

5. ARTHUR LYDIARD’S LECTURE - Simply put, Lydiard is the world greatest distance running coach.  His ideas are the gold standard for training.  This page is the transcript of a lecture he delivered in 1990 in Japan.  It is a very good summary of the Lydiard system.

Bonus Site - RunQuick - Hey, we do good work too ;-)