A google search for "Port n" usually returns good results - since there's a vast amount of information available and I have no idea what you're looking for particularly, I'll let you do the sifting.
You might also try the Internet Port Authority pages at grc.com - try
http://grc.com/port_n.htm, replacing the n with the number of the port you're after. I'm not sure how much information they have on the lesser-used ports, though.