http://www.good-tutorials.com/ This place has many useful things hidden in, it is basically a hosting place for tutorials on random stuff but you can find things on digital programs,painting, art, techniques, and other useful bits, I love it, I just wish I was on cables I'd look around on it more.
http://www.atoptics.co.uk/ atmospheric optics, its about light reflections and that sort of thing, could be useful, I look through it once when contemplating shadows and water.
http://www.guidetooilpainting.com/shadows.html Yeah, I am still looking at this myself, you may think... oil painting? But I'm a digital! HOGwash I say, besides its got useful information on shadows and other things...
http://www.polykarbon.com/tutorials/index.htm I've had this link for a long time, there's a few useful things floating around in here, and some basic concepts and techniques, it's what you can use to adapt for your own knowledge base.
http://www.conceptart.org/forums/showthread.php?t=870
(someone below linked concept already but this thread here is one particularly that you should look at, from start to finish this guy never gave up, from student to a teacher!)
I have more links floating around.. but hell they are unorganized, asto rocks because his are well.. organized... IF I find anything really useful I will edit my post and add more.
Oh! I do say if someone does find some MONITOR calibration links, post them those would be very useful for all of us who only paint digital and I can't profess how important it is that your monitor's brightness and contrast are correct, the best pages for testing that though would have to have black backgrounds though as most sites are stupid and put white around the gray scale test and all of that which interferes with your eyesight. If I find the one I used to calibrate mine I will post it.
Here may be found the last words of Joseph of Aramathia. He who is valiant and pure of spirit may find the holy grail in the Castle of Aaauuuggghhh...