Here's a (constantly evolving) collection of my favourite online pedagogical resources on various topics.
Quantum Field Theory
David Tong's lecture notes serve as a nice companion to (roughly) the first five chapters of Peskin & Schroeder. Here is a wonderful article about BRST symmetry, which begins by quantizing a rigid rotor both by conventional means and by reformulating it as a gauge theory and imposing BRST-invariance.
Daniel Baumann's lecture notes are a good source for linear perturbations in GR, inflation, and more. Parts of these notes have found their way into an online draft of forthcoming book with Liam McAllister. Rien van de Weijgaert has nice notes about linear and nonlinear evolution and formation of structures. Anthony Challinor's notes are also useful, particularly for derivations of various things. Here is a technical but very readable review of perturbation theory for large-scale structure. But, watch out for typos! A nice note by Sarah Bridle about the matter and CMB power spectra can be found here.
A good reference on statistics for the physical sciences is the PDG review article.