If you use an anti-spyware/anti-malware client on your device, then there's little doubt that you've tried Lookout at one time or another. While it is definitely one of the most well-known and popular anti-malware clients for Android, it has always had one huge shortcoming, in my opinion: lack of SD card scanning and real-time application monitoring for side-loaded apps.
That changes today, however, as the Lookout team has finally added these two crucial features to the array of others that Lookout is capable of. File System Monitor keeps a watchful eye on incoming files to your SD card in real-time, so it can catch potential "infections" before they can cause any real damage. Also, for those worried about how this will affect battery life, don't. Lookout claims it has "designed [the file system monitor] in a way that won't impact battery life," which is a good thing.
Also new in the Lookout app is Install Monitoring, which scans side-loaded apps as they're installed. Since Lookout has always scanned apps from the Play Store as they're installed, this is just an added layer of protection for frequent side-loaders.
The update is available now for both free and premium users, so hit the widget to grab it.