We live in a day and age where wireless Internet can be found at nearly every corner in our cities — even more so when you live in a tight apartment complex with multiple neighbors adjacent to your walls. This makes 'piggybacking' one another's Internet relatively easy, though when done so without permission, it may be considered unethical and even illegal.
In my complex, we have a small system of personal invitation. Whenever a new neighbor moves in (and happen to be close enough to be in the shared Wi-Fi range), we slip them a small note under their doors offering temporarily 'free' Wi-Fi so they can get their stuff set up and running as quickly as possible.
Because we change the password monthly, we suggest a small donation if they wish to continue using the shared service. If they choose to pay up, we simply add them to the e-mail list, and they receive the new password every month. It's a trust system that works fairly well, especially when everyone isn't heavy on the bandwidth.
What are your thoughts on sharing Wi-Fi with the neighbors?