Trojan-horse port lists are handy when you come across unexpected open ports. Check out the following URLs to find such lists.
SANS (http://www.sans.org/resources/idfag/oddports.php)
G-Lock Software (http://www.glocksoft.com/trojan_port.html)
Simovits Consulting (http://www.simovits.com/sve/nyhetsarkiv/1999/nyheter9902.html)
Doshelp.com (http://www.doshelp.com/trojanports/html)