Roslyn, Microsoft's "compiler as a service", has been updated with a September 2012 CTP. This release adds support for Visual Studio 2012, introduces new language features for VB and C#, and includes several updates to the APIs.
Roslyn comprises a set of APIs that expose each part of the compilation process, and make them available as a service. It was first released in October 2011, and last updated in June of this year. While previous versions worked with Visual Studio 2010, this CTP only supports Visual Studio 2012.
New C# language features have been added, including nullable types, multi-dimensional arrays, and unsafe code. VB also now has nullable types, as well as array literals, handles clauses, XML literals, and late-binding. Async is not yet supported in either language.
There is no VB.NET interactive window yet; the C# version is unchanged. (A full list of not-yet-implemented language features is available in the Roslyn forum.) The services and editor services APIs have been updated to remove some of their dependencies on Visual Studio, and the compiler APIs include several syntax updates for simplification.
The Roslyn September 2012 CTP is available directly from Microsoft or NuGet. NuGet allows developers to download VB and C# compilers and services individually, rather than the entire package.