Light Table is the extensible next generation code editor that connects you with your code. Emphasis is placed on making Light Table fully malleable at all levels to allow for a tailored experience that best suits you. Light Table has a plugin manager with a growing plugin community to provide even greater functionality and customization.
Light Table's general editor capabilities will work with most languages out there, but it has deeper language integrations (things like inline eval) for Clojure, ClojureScript, Javascript, Python, HTML, and CSS. More languages are available via plugins in the future.
Light Table runs on Windows, Mac OSX 10.7+, and most flavors of Linux.
Yes, Light Table will download updates in the background and let you know when you need to restart to get the newest version.
Light Table, with the exception of specific language clients, is written almost entirely in ClojureScript. For more detail on how it is architected, take a look at Chris Granger's blog post The IDE as a Value.