Math learning resources --> http://mathinsight.org/
Online compendium of mathematical proofs --> http://www.proofwiki.org/wiki/Main_Page
Stuff on tensors --> http://en.wikiversity.org/wiki/Introduction_to_Elasticity/Tensors
Course (Master 1) on Statistical Mechanics and Quantum Mechanics (in French) --> http://www.master.physique.upmc.fr/fr/m1_a_distance.html
Open Source Physics --> http://www.compadre.org/osp/search/browse.cfm?browse=gsss