ASP.NET AJAX, formally know as the Atlas project, was envisioned as a well to build AJAX applications in an ASP.NET style. Among other things, that means using drag and drop controls instead of hand-coded JavaScript for most use cases. What makes ASP.NET AJAX particularly interesting is the shared-source control library.
ASP.NET AJAX, the focus of Microsoft's announcement, is available as a release candidate. In theory, this means it should be stable enough to start production development against. But as always, this should be done cautiously as it may still differ from the final version.
Though not fully buying into the philosophy, Microsoft has embraced open source for portions of their projects. While the core of ASP.NET AJAX is still proprietary, the components and samples that comprise the ASP.NET AJAX Control Toolkit are being developed as shared-source community project.
Also available is ASP.NET AJAX Futures. This is a CTP of possible functionality still under development. Like all Community Tech Previews, this part of the release is to garner early opinions and feedback and should not be allowed anywhere near production code.
Available separately is a pre-release version of the Microsoft AJAX library. This just the JavaScript files needed for developers wishing to use the libraries in non-ASP environments. A click-through license agreement is required, but no personal information is collected.