Keybase is an open directory — no API key needed — so you can request maria’s key, get her proofs, and verify her identity in any software. The goal of Keybase is to let any security software be powered by usernames instead of offline key exchanges.
This looks very promising, although the fact that they have no monetisation strategy in place is a little worrying.
P.S. Jon Russel wrote more about Keybase here, on TechCrunch.