There are any number of reasons why you might want to keep doors from shutting. Maybe your toddler has a tendency to close herself in rooms with no way out, or maybe your cat needs access to your office to use the litter box. No matter what your reasons, just cut off a section of a pool noodle, and cut a lengthwise slit to open it up. Slide it on your door, and boom, doorstop.
If it's sliding off the door, a strip of velcro should keep it in place.