Well, sure, git provides an append/log-based distributed hash store. It's probably not what you're looking for, though: It doesn't have a library for efficient in-process access, so you need to spawn a git shell command per operation, with somewhat opinionated semantics. It's also GPL'd.
It's good for prototyping, though. (And I hear it's good at managing changes for source code!)
"It doesn't have a library for efficient in-process access"
Ah, via Ruby. RUBY?!??! Ah. Delightful.
YECCH
So, uh, can't we just make an efficient, well-designed, C library for accessing those data structures, write BSD-licensed wrappers as consenting adults, and get on with our lives? I'm speechless.
Is writing an append-only log data-structure system really that hard? Ok, then.