edtFTPnet/Free is one of the most popular .NET FTP client libraries available today. Robust, fast and with an extensive API, it provides a solid foundation for developers who require FTP functionality in their .NET applications. Full source code is included.
Very good. Very good documentation also. I can truly recommend it.
Update (4 Nov 2017): Caveat The version edtftpnet-2.2.3 that I am using does not handle german Umlauts like ÄÖÜäöü in file names correctly.