Light-Weight Visual Components Library (LwVCL) is a powerful, small, fast, efficient, framework of visual components that provides a rich set of customizable, flexible user interface components. There are versions for J2SE/J2ME Personal Profile, J2ME MIDP, .NET, and SWT. This provides a unified GUI framework for different platforms that provides the same API, the same concepts, and the same look and feel.
Optnik is a CLR library to help with option parsing and command execution. It is very similar to Mono.Options but has support for sub-commands, each with its own private set of options. Optnik is written in C# and runs on Mono and MS .NET (and probably on other CLR implementations too).