App to monitor what websites are visited over wifi / router?

I have some relatives staying with me while they get back on their feet. They claim to be job / apartment hunting but I get the impression they are not doing that at all and may even be using my internet for illegal activities like downloading movies. I've already received a notice from Comcast about downloading copyrighted material and discussed it with my relatives.

Is there an app for OS X that I can use monitor their web connection? I am particularly interested in what websites they are going to.

Thank you