by Petr Stastny
Free to use! No premium needed!
Listens to wifi signal and saves data into (phone storage)/wifiMap/(name od folder)/log.txt . Data are stored in CSV format (#;#;#;#$), where # represents number. Numbers are: long, lat, accuracy, signal strength.
You can upload log files to http://spam.soptik.sweb.cz/wifiMap/ , you will see all gathered data on map
(Weird domain name, right? Don't worry, it's just free hosting, I don't really want to pay for hosting and domain for such a small utility)
Rate and review within the app in the Community section.