Any app available that allows me to see what people on my network are doing?

I gave out my password for my wifi network (airport) to a few neighbors under one condition, they don't game online and/or download loads of illegal music.....not cause I care, but because I live in a rural area and have limited bandwith.

Is there a program that allows me to view how much information different users on my network are uploading/downloading?

I have noticed my net being really slow lately and before I change my password and block all of them, I want to double check the situation and see if they are actually using a lot of bandwith and downloading stuff.

Thanks.

well, my router that Verizon gave me is a Westell.
But the people are connecting to my Airtunes Express, not the westell router.