Today in #FreeSoftwareAdvent it's pandoc(1).
I author most of my prose in raw HTML (something I've done since the 90s, stemming from my appreciation of WordPerfect's "Reveal Codes" functionality), or occasionally I'll use Markdown. It's nice to just hand the file off to pandoc and get some other format (PDF, Word, RTF, plaintext, whatever).
Is the output flashy? Not really. Is it what I would get if I hand-crafted the desired output document? No. But does it let me lazily do conversions with basically zero effort? Yep!