I'm generally involved in the .NET space, but often work with companies that have both .NET and J2EE running in their enterprise.  Because of our focus on Team Foundation Server (TFS) and VSTS (Team System), when we run into Java, it's usually getting them moved over to TFS using TeamPrise.  (See Martin Woodward's blog for more on this great product, plus some very valuable insights into TFS and software development in general.)

But sometimes the companies we work with have J2EE as their primary servers.  I only recently learned of a very interesting product called Visual MainWin for J2EE that allows a company to run their ASP.NET applications without recompilation on WebSphere servers!  Now, web developers can get all the productivity, simplicity and maintainability of ASP.NET even before they convince their IT shops to move to a Windows platform!  Combine that with TFS and TeamPrise for their Java developers, and we're talking a real Microsoft foothold in hostile territory. 

Side note:  Speaking of Java and .NET, here's a Java Virtual Machine written in .NET!    It's called IKVM.NET!