Powered by Python, ascii.io from Marcin Kulik is fully open source, both client and server. You can install the client with a simple shell script:
$ curl -sL get.ascii.io | bash
Once installed, use the
asciiio command line interface to record your session, optionally logging in with GitHub or Twitter to store your recordings under your profile at ascii.io.
Check out the source on GitHub for info on usage and how to contribute.
Bonus: Nyan Cat!