One command I can't live without: textutil.

Basically it's a command-line front end to Apple's TextKit file import/export library. Works with a bunch of rich text/word processor formats, including OpenDoc, RTF, HTML 5, and MS Word. Critically, the HTML it emits is vastly better than the bloated crap that comes out of Microsoft Word or LibreOffice when you save as HTML ...

Install pandoc and multimarkdown as well and you've got the three pillars of a powerful, easy-to-use multiformat text processing system.