Yup.... there are TONS of apps for smartphones and tablets. If it can be done with a smartphone.... someone has probably made an app for it. They DO have the advantage of having the accelerometer and gyro built in and their output easily accessible. I've been reverse engineering a couple of little apps written for windows using java, vb, python or c+ that use basic accelerometer data to display an artificial horizon or attitude indicator for remote use with an rc aircraft. I've used them all but I'm not that proficient in any of them. The board I am using for prototyping is based on the MPU-6050 which is available on ebay for under $10. I have also been tinkering with a WII nunchuck and motion plus adapter. When I am done, I hope to have a small board that simply needs level mounting and a usb connection to the car pc, and of course an app to read and display the output. From there.... maybe an enhanced version with a magnetometer compass and atmospheric pressure sensor to offer compass heading and altitude as well.
The only thing I have really found pre-made so far is the Innovate RAD-1 Rollover Avoidance Device Inclinometer.