With Mainsoft's Grasshopper 2.0 you can port existing C# based ASP.NET Web applications to the Java universe. A new technology preview just came out, and the product should be finished by March. The Developer edition is free.
In collaboration with the Mono project, Mainsoft has released a new technology preview of Grasshopper 2.0. This technology allows you to use Microsoft Visual Studio to develop ASP.NET Web applications using C#, convert them to pure Java applications, and deploy them on Linux and other Java-enabled platforms. You can even use Visual Studio's debugger to attach to and debug the application remotely.
It works by compiling MSIL (the binary output from the C# or VB.NET compilers) into standard Java bytecode. The resulting pure Java deployment packages can be used in any Java EE application server such as Tomcat or JBoss. The Grasshopper plug-in automates the whole process and is fully integrated into Visual Studio.
Due to differences between .NET and Java, there are a few limitations on what .NET code can be supported. For example, you can't use unsafe code or call native Win32 APIs. See the online doc for more details.
Grasshopper is the free "Developer" edition of Visual MainWin for Java EE. Enterprise and Portal editions are also available - see this feature matrix for a comparison. Applications can be validated using the SUN Application Verification Kit (AVK) for the Enterprise, or if you use the non-free editions your applications can also be validated Ready for WebSphere Software. All editions use a mixture of open source (shared with the Mono project) and proprietary software.
The final release is expected to be available by March. For more info and downloads see the Mainsoft Developer Zone.