On stock (and jailbroken) Kindles you can send PDFs or MOBIs to username@kindle.com and have them auto-downloaded to your Kindle. Many people use Calibre[1] (FOSS, win/mac/linux) to sync/manage their ebook collection using this feature. You could easily couple it with Dropbox.
You can also use Calibre to have newspapers/magazines sent to your Kindle every morning. There's tons of recipes for scraping BBC, NYTimes, WaPo or $local_paper, rendering with custom CSS to get readable ad-free version, converting to MOBI and sending to Kindle. There has even been SaaS offerings running hosted Calibre cronjobs with user-provided recipes. Not sure if any still exist.
> On stock (and jailbroken) Kindles you can send PDFs or MOBIs to username@kindle.com and have them auto-downloaded to your Kindle.
That's no good for me because I tend to do a lot of random edits of my epub collection to fix editing errors, replace cover images with better ones, etc. The reason I want Dropbox is to automatically propagate changes like that, not just to transfer files over.
Upstream comment recommends running KOreader on jailbroken Kindle for (amongst other things) syncing with Calibre. I'm assuming this is two-way sync, but I don't know.
You can also use Calibre to have newspapers/magazines sent to your Kindle every morning. There's tons of recipes for scraping BBC, NYTimes, WaPo or $local_paper, rendering with custom CSS to get readable ad-free version, converting to MOBI and sending to Kindle. There has even been SaaS offerings running hosted Calibre cronjobs with user-provided recipes. Not sure if any still exist.
[1] https://calibre-ebook.com