probably someone who lives close to you (other side of the road?). Check what network you're using. Connect your Wireless router, and then you'll probably be able to choose between multiple networks on your laptop. What you could do is use an utility which tells you the strength of your network (it can be done with 'ZyDas', which was included with my Edimax wireless router), and walk a few meters to one side. Then check again for the strength. That way, you can find out where the wireless network is coming from. Oh, btw, you got free internet access now
And if you found out who it was, you can tell them you'll secure their network with WEP for a few $$$
