I think NETLIMITER is an answer here. You can install it on the other machines and see what they are doing and the rest is their use.
But GAMING alone can push you over the cap. Or using youtube, hulu and more. And we didn't start to include downloading!
I too was approaching the 5GB wireless internet cap and had to forbid all video sites. When you hit the cap, just turn it off or agree to pay for it.
I live with two roommates and the internet (broadband) is in my name. We have been going over our cap for the past 3 months. It seems to happen when my one roommate and I are at work. The culprit is downloading tons of stuff but I have no proof other than I know its him but I cant prove it. Since we all use the same wireless router my internet provider cant help. Is there a free program that can monitor downloads and uploads using our individual MAC addresses. I need to show positive evidence that its his machine that is doing the downloads...Please help if you can. I am running Vista home edition. Any advise would be very helpful. Many thanks Bob