Aptana just released a new project called Jaxer. In a nutshell, Jaxer is server filter which parses files and can execute JavaScript on the server. It's like a headless version of Firefox. All your client side libraries (like jQuery) will run in Jaxer—which is pretty neat.
What this means is that the JavaScript you write can be used in both the client and the server. Jaxer actually resolves one of the most complicated problems web developers face—ensuring data is validated using the exact same rules on both the client and the server. Because Jaxer is able to execute your JS code on the server, you can write one set of validation functions and use them both place.
Aptana's posted a screencast showing a simple client/server validation example using Jaxer that I recommend viewing. It's a simple example, but shows off the potential power of Jaxer.
I also recommend checking out Ajaxian's post on Jaxer as well as John Resig's post which both provide example code and give further insight.
At this point this product has a high "Cool!" and "Wow!" factor, but I really wonder who their target audience is. Hopefully I'll have some free time to play around with this sometime soon.
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