These websites are very interesting and informative. I use them for my equestrian education on regular basis.

Mary Wanless and her method "Ride With Your Mind".
This one doesn't need any introduction. I have mentioned her ideas in many of my articles.

Sustainable Dressage.
A very interesting website on biomechanics of correctly moving horse.

Classical Dressage.
A very beautiful website with wonderful pictures, insightful articles. And the name tells it all.

The Complete Guide to the Alexander Technique.
This website will introduce you to the Alexander Technique - A wonderful method of subtle changes to improve your posture and body awareness.

Barefoot for Soundness.
This website opened my eyes on correct hoof trimming and correct biomechanics of horses hooves. I have learned how to trim my own horse and his hooves have never been in better shape.

Ironfree Horse.
Another very interesting website on hoof trimming with loads of pictures which are very educational for training your eye.

Nevzorov's Haute Ecole .
I hope this one inspires you to look at your horse differently and to open your mind to new possibilities.