Mirko Mauri, Evgeny Shinder: Homological Bondal-Orlov localization conjecture for rational singularities is a cool preprint showing that the essential surjectivity part of the Bondal–Orlov conjecture for resolutions of rational singularities holds, at least if one goes to the Grothendieck group (hence it is true homologically). Nifty!
Jan Vonk: Torsion on elliptic curves is a set of lecture notes for a summer school. It takes the reader on a beautiful trip through the ages, and is a highly recommended read.
Database of Ring Theory is an interactive repository of properties of rings and modules. Highly fun, and very useful when you're teaching. It's like an interactive version of an algebra version of Counterexamples in Topology.