This has to be one of my favourite websites. Its informative, has some great articles and being American has a slightly different culture and way of looking at things. I often trawl through it looking for information, articles or just reading different contributors. I find in the USA they have a culture of really looking at riders style, how they do things and the way that the amateur can also learn.
Best of all this information is free!
The one classic that they have is the George Morris rider critique. George looks at a riders position and states what is good/bad and can be improved. A must read for understanding the most effective style in jumping.