I’m not sure how I missed this… ImageJ is image processing and analysis software in Java. This is complete, with UI, macros and plugins (a whole host of ’em!) to do all kinds of image processing. It also claims to be quite fast, desptie not requiring a native library like JAI.
Oh yeah, and it’s public domain. (Your tax dollars at work, if you’re in the US… And if you’re not in the US, enjoy it on us! 🙂