I like the Apple Find My Phone app. Since my girlfriend and I use her Apple ID on her iPhone, and my iPad, it's quite cool to be able to see when she is leaving work, or leaving the supermarket, etc. I just took a run through of WheresMyDroid, an app for Android that does pretty much the same thing (at a basic level) and then TONS more.
After installing on the device (My Samsung Galaxy S2) and walking through the simplest wizard in the entire universe you can then find the device from WheresMyDroid.com, via their Commander. It took about 45 seconds total to find me, at home, which isn't bad, seeing as at the time I didn't have GPS turned on (the Commander sends a code to the phone to turn it on) and then that had to signal lock.
I highly recommend the app.
Here is me: