Most "free" Wi-Fi spots are free for one of the following reasons:
a) The retailer is offering it on the hopes that the longer you stay in their store, the more you'll spend (Starbucks, Target, Tim Horton's, etc.). If they use a password, they usually have it posted somewhere in the store; if they don't, though, chances are they'll either ask you to 'register' your email address with them, or (worse) it's not encrypted.
b) The Wi-Fi "hotspot" is being offered by your home Internet provider (i.e. TimeWarner). It's free, but it's only free for the ISP customers, & you'll have to sign in with your account information.
c) The local government entity or non-profit entity (city government, local library branch, etc.) offers it as a...