Cryptomator provides transparent, client-side encryption for your cloud. Protect your documents from unauthorized access. Cryptomator is free and open source software, so you can rest assured there are no backdoors.
W.A.I.T. (What Am I Trading?) Cryptomator sells their mobile apps while keeping this desktop one free, except when you want to change the default theme, for which you have to trade currency to them. This is a small trade for the desktop app, but knowing that trades re a slippery slope, then we have to warn people about this app not being entirely trade-free.