I use:

• qpdf for mucking around with pdfs, reordering, selecting pages, combining them, etc.
• ffmpeg for video and audio sicing and transcoding. Usually encompassing a command in a script because I forget the precise params every time ;p
• nvim for anything like Markdown (which can be converted to other things like LaTeX or pdf or html, sometimes in multiple stages)
• imagemagick for simple image conversion stuff.
• wget for downloads ^.^
• youtube-dl or yt-dlp for grabbing youtube stuff.