Titanium Backup - Titanium Backup is a very old backup app that has been around since the early days of Android. In fact, it's one of the few apps that covers all versions from 1. Read more about Titanium Backup.
F-Droid - F-Droid is an interesting alternative for Android users who want to try something different and not have to use the Goole Play store all the time.
oandbackup - Make backups of selected apps on your device and restore from those backups.
APK Pure - Download apk for Android with APKPure APK downloader. NoAds, Faster apk downloads and apk file update speed. Best of all, it's free
SyncDroid - SyncDroid is basically and iOS and Android smartphone manager that lets the users of both smartphones to transfer their mobile content from one smartphone to another within few simple steps.
APKMirror - Android Market and APK Downloads