Xnb
From NDesk
Xnb: X protocol libraries for C#/.NET
The X protocol .NET Binding (Xnb) is a C# implementation of XCB, a replacement for Xlib featuring a small footprint, latency hiding, direct access to the protocol, improved threading support, and extensibility.
In addition to the protocol transport layer and full coverage of all current Xorg extensions, Xnb also provides a thin implementation of the Xau library and xauth tool for managing X authentication.
Xnb has not yet reached a usable state.
It can be found in Mono svn is is due to be maintained in the NDesk git repository.