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.

#### Cosmology

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.

#### Other

A good reference on statistics for the physical sciences is the PDG review article.