| A program that queries every TCP port on target host and lists all open ports. Used to see what network services the target host is running (probably in order to see if the host has exploitable security holes.)
These can be used for legal purposes too - for example, to check what ports you have open to outside. (In other words: You can do this unto you before someone else gets the same idea. =)
One of the best is called nmap. It also scans UDP and does all sorts of other things. | 
