With the prevalence of handheld wifi-enabled devices, it is only kind to open up your network for others to use. I live in an area that has some university students. Often they will hop onto my network. Fine by me if they are just doing some light surfing. Now and then one of them will be downloading big files over the network, so I simply ban their MAC address. At the same time, I guess it would be fair to filter any actual neighbouring computers out, since they all have broadband anyway, and there's no reason to use my network. An open wifi network is a gesture of goodwill towards your fellow human being. It's great to be able to check the bus schedule on my iPod touch while I'm walking down the street, or to receive email wherever I am.