A comprehensive list of LaTeX packages including documentation can be found on CTAN.
Some common packages are:
- amsmath by AMS-LaTeX
- babel – typographical rules and hyphenation patterns for many languages
- fontspec – font selection in XeLaTeX and LuaLaTeX
- hyperref – hypertext links and other PDF features
- microtype – automatic micro-typographic adjustments
- tikz – drawing diagrams