Open Source Network Servers in C#

The C# Mail Server is a C# port of the Java Email Server (JES). JES is an established email server targeted at small centrally maintained installations.

Multi-threaded FTP Program written in C#

NeatUpload allows ASP.NET developers to stream uploaded files to disk and allows users to monitor upload progress. It is open source and works under Mono's XSP/mod_mono as well as Microsoft's ASP.NET implementation. It features 2 custom controls: InputFile allows the user to select a file to upload, and ProgressBar displays the upload progress either inline or in a popup. ProgressBar even allows users without JavaScript to see upload progress.

NMail provides an SMTP client & server, POP3 & IMAP4 servers written in C#

SuperSocket is a light weight extensible socket application framework. You can use it to build a command based server side socket application (like FTP server, SMTP/POP3/IMAP4 server, SIP server, etc) easily without thinking about how to use socket, how to maintain the socket connections and how socket works(synchronize/asynchronize). It is a pure C# project which is designed to be extended, so it is easy to be integrated to your existing system. As long as your systems (like forum/CRM/MIS/HRM/ERP) are developed in .NET language, you must be able to use SuperSocket to build your socket application as a part of your current system perfectly.

Microsoft.NET WebDAV Server Framework (RFC2518 compliant) which allows users to easily expose WebDAV functionality via IIS to custom solutions