"Our goal is end-to-end encryption with minimal friction," Keybase co-founder Chris Coyne wrote in a blog post, and anyone can send a Keybase Chat message to anyone else on the internet -- even if they haven't signed up for Keybase yet. If you wanted to message Coyne, for example, you could simply enter his Twitter (or Reddit, or Hacker News) handle @malgorithms into Keybase Chat and leave a message in his secure inbox. If someone joins after you've already left them a Keybase message, the app checks for proof again and the server never sees a decryption key. If you have a couple handles from various services for one contact, Keybase consolidates them into one profile and each active device has its own private key.
Keybase also offers Forward Secrecy, public broadcast messages, command line access and an API for building bots and scriptable services. For now, the Keybase Chat allows for 4,000-character messages and attachments, and includes some basic blocking and muting features which will be necessary now that everyone on the internet essentially has an encrypted inbox waiting for them. Coyne and company promise that "more advanced blocking / reporting / nuking features" are coming in a future release.
Keybase is currently available for macOS, Windows and Linux platforms, with a mobile version launching "soon."