There are a lot of applications which do not arrive to Android market but still those are released by developers in APK format and called off-market apps.
Now, if you want to install a non-market app on Android phone then follow the instructions written below.
Install Off-market APK Apps on Android Phone
Download your desired off-market APK app on your computer.
Note: APK app has .apk extension.
Open the URL www.apkinstall.com, and then click on Browse files button and upload your downloaded APK app.
On completion of upload process the website will create a QR code for your APK app. So, scan that QR code through QR code scanner on your Android device.
Note: Scan the code after 15 minutes of uploading, otherwise download will fail.
Then click on Open browser button, so your app will get downloaded to your Android phone.
Now, tap on your downloaded app to install it and tap on Install button to complete the process.
Enjoy, you have installed a non-market APK app on your Android phone.