To further clarify - many businesses are now offering Open WiFi access in their shops / stores - like Panera Bread, Krystals, etc (for the US) and other places. If you go to such sites you can access them.
Then there is the not so nice way of doing it - War-driving - this is where you go around looking for open WiFi connections and make notes (or use an app that will make the notes for you, like location on Maps, etc.).
More than a few are doing this, but you have to realize something - if you access someone's wireless connection without notifying them, you're stealing. Plain and simple.
If the Access Point (AP) has a password / key then you have to know the key in order to use it. Trying to hack into a WiFi connection is also illegal - and beyond the scope of this thread.