Where's My Droid to the Rescue! Seriously, this App should come shipped on all Android phones. It has a few really cool features:
1. Turns on the ringer to high volume so you can look for the phone. Text the code "Wheres my droid" to the phone.
2. If you still can't find your phone it will send the GPS corrdinates back to you.Text "GPS my droid".
3. The paid version ($1) comes with a few additional features, most notably it adds a custom Button to your home screen that allows anyone who finds your phone to find your contact info.
Of course with this app, your phone has to actually be on AND lost. I love that the developer of this app is a college student just doing what he loves. I will definitely be getting the paid version of this app! He has a few instructional videos here that will help you get started.
Price: $1 (So worth it!), Free version too
Phone Tested On: Nexus One
Developer: alienmanfc6 - Brad Degelau