Open Source C# FTP Library
The title says it all. Get it here.
One thing I would say - the developers have a bit of a curious naming convention in their library. I think they've tried to be true to the FTP commands that one might use from a command line, rather than OO-ing it. For example, there's a method called "User" for setting the username, rather than a property.
Otherwise, seems to work fine. Has source code if it doesn't!