Wi-Fi Analyzer is a free app which displays the strength of all Wi-Fi hotspots nearby.
It's been out for a while now, however Recombu's review of it brought the app to my attention. Shown on a bar graph with varying colors, each Wi-Fi connection in the area is displayed along with the "channel rating," which will give you some idea of how many people are connected to that signal. In the picture shown, the green signal is the lowest of all—which means it's also the busiest. The red arc is the highest on the graph, which is the one you should be connecting to.