The latest release of Telerik's RadControls framework for ASP.NET AJAX has a really cool feature: RadCompression. Telerik provides a HTTP module that compresses your JavaScript files, Silverlight applications, and AJAX/web service requests. While it may not work with every request (it doesn't support web service page methods and it doesn't support certain application types), it does work really well in what it does.
We added this to our application in our development environment, and noticed that it did improve the web site speed on our local machine by up to 20% (as a rough estimate from visual tests and not actually performing benchmarking).
It's very easy to add; all you need to do is add the reference to the module in the config file, as documented here: http://www.telerik.com/help/aspnet-ajax/radcompression.html