Is There Any Free Heat Map Software?

There's some free heat map software out there, right? I want to walk around with my laptop and look at the visual representation of my WiFi. If it can display signal strength on a floor plan, that is a form of creating a heat map from that device's perspective. On a Mac, I use WiFi Explorer a lot. There's also on a Mac tool called AirTool and WiFi signal all by the same guy.