While there is no shortage of security apps on the Play Store, aeGis one stands out a bit for a few reasons. For starters, it's dead simple to use. Set up a specific trigger phrase and you can text your phone to lock the display, remotely wipe, find the address of, or sound an alarm from your phone. There's no web interface, unfortunately, but this app trades the elaborate suite of services of something like Avast for simplicity.
What's even cooler is this app is entirely open source. Built by a member of the CyanogenMod team as a side project, the entire codebase is available on Github here. The app builds on SMSAlarm by adding the lock, wipe, and locate functionality. If an enterprising developer would like to improve it any more, they're free to do so under the GPL license. Naturally, aeGis itself is also free.
Everything except the alarm functions are disabled by default, so you don't have to worry about a mischievous friend or malicious enemy screwing up your device. You can set a custom phrase to trigger any of the app's functions, and are highly encouraged to do so. Leaving the triggers as the default is a great way to leave yourself vulnerable.
Update: I reached out to the developer and he explained that this app is U.S. only for now due to the way it handles addresses. When you text your phone from another device, it will reply back with a street address that it figures out from latitude and longitude coordinates using Geocoder. However, this only works in the U.S.. Once the dev figures out a way to support location for other countries, compatibility will be updated.
Update 2: The developer reached out to us to let us know that the app should be available internationally now. For international users, texting the phone to return a location will return coordinates, but might not return a street address. So just keep that in mind. Still, the information should be useful.