Java to C#

http://www.developer.com/java/other/article.php/2235411

The Java Language Conversion Assistant (JLCA) is a tool that provides the ability to convert Java code and library calls to Microsoft Visual C# .NET. Built based on the ArtinSoft migration technology, this second version adds a number of additional features to its abilities. This includes the ability to convert code from both the client and the server. Developers will be able to convert Web as well as Windows^®-based applications. Java Server Pages and Java servlets can also be converted to ASP.NET.

Java to C

Harissa

http://www.usenix.org/publications/library/proceedings/coots97/full_papers/muller/muller_html/node3.html

Harissa is a Java environment that includes a compiler from Java bytecode to C and a Java Virtual Machine integrated in a runtime library.

C to Java

http://theory.lcs.mit.edu/%7Eedemaine/papers/CPE98/paper.pdf

Emphasize on Pointer to Reference type

.NET language to Java

Project Tango: Java and .NET Web Service Interoperability

http://java.sun.com/developer/technicalArticles/glassfish/ProjectTango/

Project Semplice: Visual Basic for the Java Platform

At the heart of this early stage technology, is a Visual Basic to Java-bytecode compiler.

http://blogs.sun.com/roller/page/tor?entry=semplice_keynote_demo

NET2Java: Translating .NET source code into Java source code

https://net2java.dev.java.net/

http://blogs.sun.com/roller/page/dannycoward