For a cheap demonstrator/toy, there's a free bubble-level app for Android phones that have the appropriate hardware. I'd say it's accurate to a minor fraction of a degree.
Yes I've got that app and a couple of others like it to help me do fabrication in the workshop.
This one's pretty handy at times -
https://market.android.com/details?i...=search_result
Edit - This looks interesting as well -
https://market.android.com/details?i...e=related_apps