I think people on tumblr (not everyone, probably, but plenty of people) should keep a tag called "#endorsed software" where they talk about software that they use and recommend. So here's one of mine.
I use this delightful little tool called SyncThing to synchronize files between my various devices. There is no "storing files in the cloud" with SyncThing, it only stores files on computers I own, such as my smartphone. Whenever any two of my devices are connected to the same internet access point (eg my home wifi), they find each other and make sure they both have up-to-date copies of all the files I sync.
You can set up different folders to sync, and each folder can have its own rules about how sync works. I have one folder with regular markdown text notes that is synced to all my devices, so I can write down notes anywhere. I have another folder that automatically syncs music I place in it from my desktop to my smartphone, this is only synced between my desktop and phone so that it does not take up space on my tablet.
SyncThing automatically detects if you make different changes in different places before they synchronize, and keeps both versions so you can combine the changes yourself. There are configurable settings on how long to keep older versions.
SyncThing is both free as in zero dollars, and free as in free speech. Anyone can take a look at the software, make a tweaked version for their own use, and submit bug fixes to the main project to improve it for everyone. It is available for Windows, Linux, Android, Mac, and possibly iPhoneOS.
