Some of the things I do regularly:

• vi/emacs: filter parts of the edit buffer through commands (perl one liners, tools like fmt(1), specialized little tools for building indexes, cross-references, and the like ...)
• vi/emacs: map a key to the compile command (or the respective vi macro) to run Makefile entries that do compilation, testing, publishing to web sites, ...
• emacs: use the shell mode to record all your activities, e.g., when working as root on other's machines, or when doing a new installation routine, ...
• vi/emacs: make the editor put in templates for common tasks
• vim/emacs: develop your own, intelligent (for you) syntax colouring scheme
• emacs: use the editor as shell substitution on platforms where you have no convenient shell (command line history, ...)

Christian Lemburg
Brainbench MVP for Perl
http://www.brainbench.com