A week late. Whoops.
Leonid Positselski, Olaf Schnürer: The inclusion of small modules into big ones: Is it fully faithful on unbounded derived categories? is an interesting preprint discussing the features (or pathologies) of derived categories one needs to be aware of. An often used method is to describe a subcategory of the derived category, by restricting the cohomology of objects that you consider, instead of considering the derived category of a subcategory of the abelian category. This preprint explains when you can do this, and also when you can not do this.
Giovanni Cerulli Irelli: Three lectures on quiver Grassmannians makes for some nice reading material on quiver Grassmannians, with many explicit examples. I like explicit examples.
Reflections on AGONIZE and online conferences is an interesting insight in Daniel Litt's experiences with running an online conference.
biblatex2bibitem is a very convenient package if you are like me, a stubborn advocate of biblatex and biber, in a world where arXiv uses a version which is incompatible with TeX Live's latest release.