Port 17, "Quote of the Day" protocol; when a socket is initalised and connected to this port on a server machine running "Simple TCP/IP Services", it will respond with one of those quotes.
This is mainly historic, Alot of operating systems do this.
Other common ports include Echo (Port 7), Chargen (Character Generator - Port 19), daytime, discard, and many more... Check
http://www.xmas-i-am.com/video/Comon%20Port%20numbers.txt for more information (1.2MB File - warning to modem users

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Microsoft havent wasted time programming this, its included as a bolt on for most OS's, for simple diags and historic reasons. They can be disabled and uninstalled (Simple TCP/IP Services in Networking Serviecs in Win2k, I believe).
Good fun to play with, you can knock your own Simple Port server together like this in under 20 seconds