A few months ago, Liam spent some time with 17 mobile security apps, one of which was an app called Cerberus. He came away quite impressed - so impressed, in fact, that he crowned it the winner (and, as a license is just $4, it was also crowned the best for the budget-minded).
The app is always free to download and comes with a 1 week free trial, but a lifetime license is normally $4. I say normally because, as you have likely deduced from the title, the developer is offering free lifetime licenses in celebration of hitting 100,000 users.
To get the license, just download the app and register an account (it requires a username, password, and email address - nothing tricky), then fill out the form and hit Submit.