It's a shame that keysafe[1] wasn't mentioned. It's a fairly interesting project, where it splits up the key into separate parts (using Shamir secret sharing) and it's encrypted and sent to separate servers. Brute forcing is quite hard because even with the right password it takes more than an hour to recover your key (due to the difficulty chosen for the Argon2 hash). Joey Hess gave a talk about it at LCA[2].
I use SSSS to store physical copies of the key I use to encrypt my backups with ssss http://point-at-infinity.org/ssss/ . Basically print a bunch of copies and give them to people I trust.
[1]: https://keysafe.branchable.com/ [2]: https://www.youtube.com/watch?v=AYegjnDWvww