• Resizing images: mogrify (part of the imagemagick suite)
• ffmpeg
• pdftk is king for rotating/cropping/appending pdfs
• LaTeX everything
• make/shell - to script/automate image and document editing
• pandoc is reasonably good for many things
• latex2rtf - to get plain text for word counts out of LaTeX source
• wc - word count, line count
• ispell -t - does spell check in the terminal. The -t is so that it'll mostly ignore LaTeX commands in the source

I'm sure there's more but I don't memorize them, they kind of get remembered when I need them.