Android: Retail stores are tracking your movements with your smartphone. While you could stop this by turning off Wi-Fi every time you leave the house, an easier solution is AVG's free PrivacyFix app.
In addition to auditing and protecting your social media accounts on your phone, the new Wi-Fi tracking protection stops your phone from being tracked when you're out and about. You can authorise specific locations where Wi-Fi will be enabled, and everywhere else the app will block your phone from transmitting its unique MAC address, which retailers are using to track your movements.
Because there's little (or no) transparency about how this information could be used, if you're concerned about your privacy, head to Google Play to download AVG PrivacyFix. (Although the app is available on iOS, the Wi-Fi tracking protection feature isn't available yet.)
Even if you don't care about the privacy part, turning off Wi-Fi when not in use will help with your phone's battery life.