(In its breadth, Torah addresses virtually every life issue that arises. Here’s a particularly modern one, looked at through the lens of tradition. It’s a good example of what Rabbi Dovid Rosenfeld described as sages looking at the eternal meaning of Torah, then relating it to a specific generation or a current circumstance. It might appear to be “legalism,” but correctly understood, it’s applying eternal truth — Torah — to circumstances not directly mentioned in Torah itself. Thus, every question and every concern is ultimately a matter of worship.)
Can I Use My Neighbor’s Wi-Fi Without Permission?
Yehuda Shurpin 
I work during the day, and because I mostly use the Web at work, I don’t have an Internet connection of my own at home. Sometimes at night I like to check my e‑mails and browse the Internet. One of my neighbors has an open, unsecured wireless connection. Can I use his Wi-Fi without permission?
Before beginning a discussion about using unsecured Internet connections according to Jewish law, it must be noted that many countries and states have specific laws that prohibit accessing someone else’s computer or network without authorization. According to Jewish law, as long as civil law does not contradict Torah law, “the law of the country is the law,” and the halachah follows civil law. In New York State, for instance, unauthorized use of a computer network is considered a Class A misdemeanor.
Therefore, any discussion on this topic from the perspective of Jewish law must be based on the assumption that civil law does not explicitly address it. Nevertheless, the question of using an unsecured network raises a number of halachic issues, and is worth delving into further.
Borrowing Without Permission
When you use your neighbor’s Wi-Fi, you are essentially “borrowing” his computer, router, modem and Internet connection without permission. The Talmud records a dispute among the sages as to whether one who borrows something without permission is considered a borrower or a thief. The halachah follows the opinion that this person is considered a thief, even if he or she intends to return the object afterward.
There are exceptions to this rule, however. In cases where the object of “theft” is something that no one (not even a small number of people) would object to someone’s borrowing without permission, and there is no risk of damaging the object, halachah considers it permissible to use it without approval.
Although accessing the Internet through an open Wi-Fi connection rarely causes damage to the network, it seems safe to assume that at least a small number of people would object to their Wi-Fi being used without permission, and doing so would be considered stealing.
 for complete article, see: http://www.chabad.org/library/article_cdo/aid/2592560/jewish/Can-I-Use-My-Neighbors-Wi-Fi-Without-Permission.htm