A few months ago, I released the Open Claiming Protocol, in order for an interoperable, open standard for people cryptographically signing things. I wonder, how does this overlap with that, and could it be useful to support it? I agree it's much more general purpose though...

http://openclaiming.org/

I know that there are some sites, like proofofexistence.com that acts like a notary service anchored to Bitcoin. Is that similar?

To me, it seems that there are some secret things you might want to hash and sign, and timestamp, like agreements that you can prove that both parties signed them, etc.