Firstly, I just want to state that I don't condone doing this becase it's illegal. It just happened to me by fluke
As you may or may not know, I live in an apartment.
I was setting up the wireless adaptor for my new PC (I don't need it ATM, but I thought I'd set it up so it's ready when I do want to use it). When the instlaller completed it automatically searched for network connections and it not only found mine, but it found someone else's too o.O. I have an SMC router, but my list showed "SMC, signal strength 98% Lynksys, signal strength 42%"