Mac/Windows: If you keep your iPhone in your pocket all day long, it's easy to lose track of the battery level. iBetterCharge is a desktop app that keeps an eye on your iPhone's battery level and alerts you when it gets low.
iBetterCharge watches any iOS device connected to your Wi-Fi network that has the "Sync this iPhone over Wi-Fi" option enabled. From the menubar, you can instantly see how much of a charge you have left and set up notifications for when the battery gets low. Assuming you don't want to keep your iPhone plugged in all the time, this iBetterCharge is a simple way to keep an eye on your battery.