Specification
Get the C# 4.0 specification from Microsoft: http://www.microsoft.com/download/en/details.aspx?id=7029
Mono
The mono project provides C# tools and runtime for the Linux platform. You can use mono and mono-develop (An IDE similar to Visual Studio) to develop and deploy C# applications. http://www.mono-project.com
Mono-Develop
Mono-develop is an IDE for the mono project. It provides an IDE similar to Visual Studio. It is capable of working with Visual Studio projects and is a project, along with mono that was a joint venture between Novell and Microsoft. http://monodevelop.com
GTK#
GTK# is a set of C# bindings to GTK. The complete Microsoft Winforms API has been implemented so that Winforms applications are completely portable across the Windows and Linux platforms. GTK# is provided with mono-develop so that you can start creating GUI applications straight away. Cool! http://www.mono-project.com/GtkSharp
Python Integration
Whether you want to access .NET assemblies from python, or enable an application for python scripting the functionality is bound to be provided by the python.NET project which can enable .NET integration for an already installed python version, or as a standalone package. http://pythonnet.sourceforge.net/
Embedded
Microsoft provide the .NET micro framework for using .NET applications in embedded applications. http://www.microsoft.com/netmf/default.mspx
The easiest way to get embedded is to use a design that has already had NETMF ported to it. Porting NETMF is not the easiest thing in the world! Tiny CLR provide several boards and a good community of open source examples and drivers. The CLR + support firmware eats ~50k RAM and ~400k Flash.