You clearly got the
WPA password of one of your neighbor wireless
network (as guessed Scott).
Since this is illegal, and may compromise you,
you should locate this neighbor (this is another
interesting security topic per se), and inform him.
You will have to inform your neighbour that you accidentaly got
his network password, that the technic he is using to secure his network
is false security (as usefull as an empty extinguisher). That his technic
WPA is easily broken by any neighbor within seconds. Then his
computer and his network traffic are completely stripped bare.
To stay accidentaly the owner of your neighbor wireless password may compromise you in the same way than keeping the keys of his appartment
may compromise you. Let's imagine for a while that this neighbor is a criminal torturing children in his
appartement and broadcasting snuff movies on the Internet. To exhibit
that you have an access to the network of this neighbor may let him defend
on the ground that his network was hacked:
"See, this is not coming from me. My network is hacked and used by this neighbor. All his traffic is coming from my IP address. He did it to hide behind my network access. This is clearly your guy."
The only real security on wireless networks has a completely different kind of name. It's
802.11i. All the other commercial names starting with
Wi-Fi are just empty extinguishers.