I've searched around and found that this fork of SharpSSH https://bitbucket.org/mattgwagner/sharpssh and SSH.NET https://github.com/sshnet/SSH.NET are the most up to date and best maintained libraries for SFTP http://en.wikipedia.org/wiki/SSH_File_Transfer_Protocol (not to be confused with FTPS http://en.wikipedia.org/wiki/FTPS) communication in .NET. SSH.NET is a clean .NET 4.0 implementation of the SFTP protocol, and I've used it in a couple of solutions with flying colors and great success.
The original SharpSsh http://sourceforge.net/projects/sharpssh/ seems to be dead and most other solutions either require installation of Windows executables or a bucketload of cash (or worse; both).