Very cool! I like the presentation on the landing page a lot. It's immediately apparent what it does.
The most difficult part of using this for me seems to be trusting tmate--not that tmate is inherently untrustworthy, but IMO untrustworthy by default is a good policy.
Well, the code is open source so you can go over it and compile from source if you want. The server is also open source, so you can run your own if you're concerned about the public server.
