Got this from the Windows XP mailing list maintained by Sunbelt Software (great mailing list, BTW — there are lots more here). If you’ve got a port that’s open or being listened to on your system, chances are you can find it at one of these sites.
http://andrew.triumf.ca/cgi-bin/port
http://www.neohapsis.com/neolabs/neo-ports/
http://lists.gpick.com/portlist/lookup.asp
http://www.snort.org/ports.html