As far as my IDE, I do all of my web development in ActiveState's Komodo IDE. It touts itself as the premier IDE for dynamic languages (as well as web languages like HTML and CSS). I've used it for Javascript and Python progrmaming, and am very happy with it. It has built-in integration with all the major version control systems and a very simple project interface that I really like (I always hate how much Eclipse's project manager gets in my way every time it tries to help me).
For coding, it has all the things you'd normally expect, syntax highlighting, errors and warnings, and code completion. It has built in library files for jQuery and a bunch of other libraries, but not for Closure. However, if you point to the Closure directory in the library settings, it'll scan through and you still get pretty good auto-complete and code hints. The full-featured IDE isn't free, but there is a no-charge version called Komodo Edit, but I haven't tried that.
Other than that, I highly recommend Plovr for any serious Closure development (as you can tell if you've gotten this far in the tutorial).