Here's a list of some of the web sites I've bookmarked that might be of interest to you. Also, I'll mention a couple of books I recommend reading.

Links:
What is a Socket?
Beej's Guide to Network Programming Using Internet Sockets
Unix Socket FAQ
TCP/IP FAQ
Winsock Programmers FAQ
TCP/IP Resource List
Sockets.com
SockAddr.com
FTP RFC 959
SMTP RFC 821
Richard Stevens Homepage
Internet Assigned Numbers Authority
American Registry For Internet Numbers

Books:
Effective TCP/IP Programming by Jon Snader
Various books by Richard Stevens

Feel free to add yours to this thread, if its popular enough, I'll make it a sticky.