Android: Wi-Fi Analyzer can scan nearby wireless access points and show which spot has the best signal and least traffic. That's neat on its own, but the free app can also help you boost your home wireless network.
Wi-Fi Analyzer does a pretty nifty job of offering up intriguing views of your Wi-Fi connection. Pictured at right is the loop graph, which does a good job of showing which nearby hotspot offers the best connection possibilities. In another view, you can see which channels on the router you're connected to would offer the best connection, in star ratings. Gizmodo commenters and other reviewers have seen some decent improvements in their Wi-Fi networks by switching to a better channel, though you should note that channel switching might muck something up, potentially, and a router reset might be in order. All said, though, it's a pretty handy tool to have on your phone.
Wi-Fi Analyzer is a free download for Android phones only. A complementary app, Wi-Fi Connector, is needed to connect to Wi-Fi spots directly from the Analyzer, though that's a free download as well.