Mac: NetSpot is a free utility that makes it easy to map out your wireless network, do a site survey if you have a particularly large one to manage, and to visualise where signal is strong and weak, and what might be getting in the way. We've mentioned NetSpot before, but the app has come a long way since then. For one, the app fully supports Wireless AC routers and access points, makes it easy to manage those access points if you want, and includes a channel and strength analyser that can run in the background. One click and you can see how strong your network is, and watch it in real time as you move across your house or apartment, or between rooms.
The app also builds a live heat map of your network (or the network you're connected to -- you don't have to use it on yours, obviously) so you can see the strong and weak points, ideal if you're somewhere where the network is spotty, or you can't pinpoint where the access points are physically located, but you are free to move around. You also get built-in speed tests, support for multiple networks so you can see how your Wi-Fi and your neighbour's overlap (and if there's channel interference), and more.
NetSpot is completely free, but there are professional versions with more features for network administrators or companies looking to outfit their admins with solid survey tools. You can read the differences here, but the free version will be fine for most people. Hit the link below to check it out.