There are many free mobile apps available for measuring Wi-Fi signal strength. On Android, the most popular free app is Wifi Analyzer. On Apple iOS, there are various free ad-supported apps such as Network Analyzer and Wi-Fi Finder by JiWire Inc.
The most popular mobile apps that measure Wi-Fi signal strength tend to either be paid or ad-supported. In addition to mobile apps, there are also signal strength utilities such as Acrylic WiFi for Windows or the in-built network diagnostic tools in Mac OSX. These can be useful on a laptop computer for tasks such as hunting for Wi-Fi dead zones around the home.