 <p>Cross-platform generally refers to a technology which can be used for multiple operating systems. For example, Mono is an open-source implementation of the Common Language Runtime (CLR), which are the underlying libraries required by .NET.</p> <p>Mono runs on Linux, BSD, Unix, Mac OS X, Solaris and Windows. Mono itself is not an IDE, but several cross-platform IDEs exist as well. The most popular is <a href="http://monodevelop.com/" rel="nofollow noreferrer">MonoDevelop</a>.</p> <p>Several languages are built on top of the .NET framework such as C# and VB.NET. C# is the most popular for cross-platform development.</p>