AVG AntiVirus PRO is £8.99 on Google Play, but just 59p over on the Amazon Appstore for Android. I think that’s what they call a “no-brainer”.
Includes AntiVirus, Battery, Data and Storage management, Task Killer, device location, App Locker, App Backup, and Call & Message Blocker.
Requires at least Android 2.1 and 9.7MB of space.
(Thanks to brownbear86 at HUKD)