NetBeans support for Google App Engine - the title says it all. I quite like NetBeans, by the way. It looks quite bare (not particularly well integrated into Cocoa - Eclipse makes a much better visual impression), but unlike the alternatives, the plugins seem to work quite well (Eclipse produces strange errors, IntelliJ requires you to find the right version of the plugin for the right version of the IDE). And the Clojure plugin for NetBeans seems to be the nicest so far - the REPL is really good.