Android: Google Play blocks some apps from being installed on your Android device, whether for phone incompatibility or region-locking. APK Downloader Extension bypasses those restrictions, and is easier than side-loading.
This extension works with any Android browser. When you come across a restricted app, open it in a browser on your phone or tablet. Go to the Share menu, select APK Downloader Extension, and at the next screen, tap the "Get" button. That's it -- the app will be downloaded to your phone.
APK Downloader Extension requires Google Play Services to work; if you don't have that installed, you will need to log in with your Google Framework ID. The app's settings also have an option to auto-install anything you download.
APK Downloader Extension does not work with paid apps -- it will only download free apps. It's easier than side-loading an app, and since the app is from the Play Store, you don't have to worry about the security risks involved with third-party APKs. That said, caution with unapproved apps is always wise.