Use it as a wireless router for Internet: With the help of built-in Wi-Fi Hotspot feature you can easily use an old phone as a portable router. Plug in a 3G SIM card and choose a data plan with a good data limit. You can then carry the phone around in your pocket and connect all your other devices (laptop, tablet and phones) instead of using individual SIM cards for each device.

Use it as media player on your TV or music systems: If your smartphone has audio out and TVout then you can easily convert it into a flash based media player for the TV. Get a high capacity memory card for your smartphone and copy your movies and music onto it.

Use it to test new apps and games: The number of apps and games are growing exponentially on each smartphone platform. However, it is not always easy to identify which apps are actually useful for you. Use your old smartphone as a test device to check out new apps before installing them on your main smartphone to save your device from bloatware.

Use it as a standalone GPS navigator:
Google Maps and navigation is free on your Android phone and you've probably used this while driving. If you prefer, you can also get Maps by MapmyIndia — in many situations. This means that with an old smartphone, you don't need to buy a standalone GPS navigator for the car — just keep your old phone permanently mounted in the car.

Use it as a wireless security camera or baby monitor:
There are a number of free apps that can convert your smartphone into a wireless security camera. Try IP Webcam for Android or iVigilo Smartcam for iOS — these apps use your phone's camera to stream live video that can be viewed in any web browser, any video player that supports streaming or on another smartphone/tablet. You only need to set up the smartphone in place, plug in the charger, install the app and make sure that it stays connected to your Wi-Fi network.