VsVim (GitHub: jaredpar / VsVim, License: Apache 2.0) by Jared Parsons is an extension for Visual Studio 2010 and above that adds a Vim emulation layer. It can be downloaded and installed from Microsoft's extension site, and requires Visual Studio to be restarted first before use.
Once it's installed you should be able to use Command-line mode, Normal mode, Insert mode, and Visual mode. Visual block requires some settings changes, as outlined in the VsVim FAQ. There is even beta support for the extremely popular ReSharper tool.
This project was sent in by Tiago Margalho, a C# developer who wishes he was using Vim.