Stretching and Flexibility - Written by Brad Appleton, this guide to stretching and flexibility is the definitive source for useful information.
IFA Fitness - International Fitness Association. This site offers paid certification, but I keep it around because it has some useful information online for free.
Century - Century has absolutely everything you could possibly need for the martial arts. Sometimes, they even have the best price. I always look here, but I always shop around, too.
Discount Martial Art Supply - Discount Martial Art Supply has a lot of stuff available for mail order. Sometimes their prices are unbelievable. They also have lots of weapons and knives.
Golden Tiger - Golden Tiger is the other place I always look when I need something. They often have stuff on sale and you can sometimes get a great deal.
Zen Stories to tell Your Neighbors - The best collection of short Zen stories I have found on the internet. Eac story is also accompanied by viewer comments with various levels of insight/enlightenment.
The Art of War - Sun Tzu's The Art of War is a classic text that should be read by all those who study the martial arts. I'm not entirely sure why, but you will definitely find it comes in handy if you have to lead troops into battle. Seriously, though, just read it. It's not that long.
Confucianism - A collection of books from the Internet Sacred Text Archive related to Confucius and his teachings.
Warm-Ups and Drills - All of the warmup exercises and drills. Also includes the 10 point breathing system.
Stretches - A full stretching routine.
Combinations - Combinations 1 through 27, step by step.
Pinons - Pinons 1 through 5, step by step.
Katas - Katas 1 through 5, step by step, and many other katas that I have learned, also step by step.