iBetterCharge is a free utility made by the software house Softorino that you have to install on your Mac. After that you have to pair your iPhone when connected via USB, then you can unplug it. iBetterCharge then runs in background and monitors the battery status of the iPhone. When it reaches a specific level, you get an alert via Notification Center so that you can charge it.
You don’t have to install anything on your iPhone, since the pooling happens with the Wi-Fi sync protocol you have to choose inside iTunes. I have to thank Brett Terpstra for pointing this out over App.net,