While looking for a free POP3 component for a personal project, I ran across a very, very impressive protocol library:  The Indy Project.  I can't believe I haven't seen this before; apparently it's been in development since 1993 and they've recently added native .NET support.

From the site:

Internet Direct (Indy for short) is an open source Internet component suite comprised of popular Internet protocols. Indy is the most comprehensive socket library available for any language and contains support for TCP, UDP, ICMP, clients, servers, SMTP, POP3, IMAP4, HTTP, NNTP, FTP, and many more protocols. In fact Indy implements more than 120 protocols and Internet standards!