I use Torque Lite and MyGreenVolt. The latter is obviously optimized for showing Volt specific info. Torque can show more information, but isn't as optimized and more difficult to setup for Volt specific info.
I leave mine plugged in, but I haven't used Onstar for anything but status and remote commands.
I leave mine plugged in, but I haven't used Onstar for anything but status and remote commands.