Free Zone is an Android app with the largest database of free and open WiFi networks that I found. While I've seen other apps/sites, like Wlanmap which focuses mostly on Germany and Austria and have more detailed data, Free Zone covers a much larger geographic area and has a decent number of hotspots to display.
It allows caching of hotspot data, which is useful when you are travelling to a new location.
The content is submitted by the users so there's plenty of sources.
Free Zone - Free WiFi Scanner - Android Apps on Google Play