Open Source C# Code That works on Mono and/or .NET
Java For Mono and .NET
IKVM.NET is an implementation of Java for Mono and Microsoft .NET. It includes the following components: A Java Virtual Machine implemented in .NET, A .NET implementation of the Java class libraries and Tools that enable Java and .NET interoperability.
A cross-platform UI toolkit for creating desktop applications with .NET and Mono
Xwt is a .NET framework for creating desktop applications that run on .NET and Mono (on Linux or Mac) from the same code base. Xwt creates an engine at runtime for the underlying platform and supports. 1. Windows: WPF engine, Gtk engine (using Gtk#). 2. MacOS X: Cocoa engine (using MonoMac) and Gtk engine (using Gtk#) and 3. Linux: Gtk engine (using Gtk#).