A web-based IDE for automated testing [1] with real-time collaboration features. Pretty cool for a few reasons: First, it's actually making creating automated testing suites enjoyable. This is usually a pretty unenviable task. The IDE is something I actually like using though, and have found myself wanting to use it on just about every consulting gig or website I've touched since we started on it - even ones without an explicit automated testing requirement.

Second, the technology to pull this off requires a lot of different pieces, in different environments and languages. It's been a really satisfying technical challenge to make everything work together seamlessly and automatically.

[1] http://f14n.com/