Android only: When your phone is scanning for potential Wi-Fi connections, it's using battery power it doesn't need to. Free app Y5 Battery Saver periodically checks your location, without GPS, and turns on Wi-Fi if you're home or at another previous connection.
Y5 watches as you connect to Wi-Fi spots from your phone, then memorises their rough location. As you get away from those Wi-Fi locations and Y5 notices through mobile signal triangulation, it turns off Wi-Fi, saving you power and those lightly annoying "Wi-Fi Networks Available" messages. When you're close to your trusted Wi-Fi again, Y5 turns your Wi-Fi back on.
We'd have to give Y5 more of a field test to give it a no-qualifications endorsement. Still, it's a nice utility to have handy if you like to use Wi-Fi when you can, but don't want to randomly try and connect wherever you can while you're out. You can grab Y5 from the market using the scan code at right or download from its home page.