These are some of the Emacs features I use the most on a day to day basis (plus a few tricks).

This program, given a source file, produces a document with syntax highlighting.

A Perl/Tk-based GUI for easy access to package documentation for TeX on Unix platforms; the databases it uses are based on the texmf/doc subtrees of teTeX, but database files for local configurations with modified/extended directories can be derived from them. Note that texdoctk is not a viewer itself, but an interface for finding documentation files and opening them with the appropriate viewer; so it relies on appropriate programs to be installed on the system. However, the choice of these programs can be configured by the sysadmin or user.

It takes years maybe decades to master the commands available to you at the Linux shell prompt. Here are 10 that you will have never heard of or used. They are in no particular order. My favorite is mkfifo.