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