There are two known public C# libraries for using CouchDB:
Divan
The git repository at http://github.com/foretagsplatsen/Divan has a relatively complete C# library for CouchDB using Newtonsoft.JSON and NUnit as its only external dependencies. Divan is actively maintained and in use at Foretagsplatsen (a swedish company using it in their core system). Being the author of Divan I claim it to be much more complete than SharpCouch 
For more information see the README.
git clone git://github.com/foretagsplatsen/Divan.git
SharpCouch
The project at http://code.google.com/p/couchbrowse contains a simple wrapper library for CouchDB, called SharpCouch, and a GUI client which makes use of the library. The GUI client code should serve as a good usage example, although the wrapper class is documented and fairly self-explanatory anyway.
You can get the source by issuing the following SVN command:
svn checkout http://couchbrowse.googlecode.com/svn/trunk/ couchbrowse
The project was built with SharpDevelop 2.2, but should work out of the box with Visual Studio 2005. Getting it to work with MonoDevelop should be reasonably easy, but has not been tried yet.