[edit] Overleaf guides

[edit] LaTeX Basics

[edit] Mathematics

[edit] Figures and tables

[edit] References and Citations

[edit] Languages

[edit] Document structure

[edit] Formatting

[edit] Fonts

[edit] Presentations

[edit] Commands

[edit] Field specific

[edit] Class files

We use TeX Live 2016 on our servers, which provides about 4000 packages, including:

For more details please see this blog post.

Also note that you can upload .sty and .cls files for other packages, for example those from CTAN.