Tresorit. It has linux cli client: https://support.tresorit.com/hc/en-us/articles/360009330614-...

Spider oak?

https://spideroak.com/

It's important to note though that their client is not open-source, so if one goes through all that trouble to use end-to-end encryption, it seems a bit unsatisfactory to me to then trust this company to actually keep the private keys on my machines (and encrypt things correctly).

Personally I used syncthing which doesn't do encryption but also only uses my own devices, so I can keep the data on my machines at all times.

In the past I used seafile which does support encryption (and it's self-hostable): https://www.seafile.com/en/home/

Tresorit is nice, but seems kind of pricy. Boxcryptor works well on top of Dropbox. Personally, I've switched to Nextcloud which keeps everything in my control, and it also supports e2e (though the e2e UX is still rough around the edges).

I think syncthing wins here. Not at rest, mind, but since you control the endpoints that should be okay.

Even though it's proprietary, Resilio Sync does encrypted peer to peer sync, plus it allow encryption-only nodes. These contribute bandwidth, but cannot decrypt the data.

I use a cryfs mount stored in Dropbox. It's been quite painless.

There is restic