Another interesting IDE under development is a project funded through Kickstarter.com, the Light Table IDE:
http://app.kodowa.com/playground
The IDE is heavily inspired by the concepts Bret Victor presented at CUSEC 2012: Inventing on Principle
Chris Granger felt so inspired by Bret's ideas, that he quickly put together a proof-of-concept for a new Clojure IDE, which he calls Light Table. The project got $300k of funding through Kickstarter, and very early releases of the IDE are available through the project playground. Installation is as easy as a single download and click, if you have Java and Chrome installed on your system.
The IDE is in a very early stage, but has some very distinctive features, especially the "live" view of your code in the right panel. Check this screenshot of Light Table running in Chrome: