This technique also works very well for video. The photographer rigs the video camera on a ball head so it can be flipped back into the upright position and then walks down a path with the camera running creating a ground level movement effect.
Monopods are also useful for working with off camera flash. Many shoe mount flash units come with small stands that can be mounted on tripod screws. These strobes can be triggered through one of several methods. The easiest is with a wireless flash controller such as Canon's ST-E2 or through Radio Slaves, Pocket Wizards or other remote triggers. Other options include connection via off-camera shoe cord or even via photoelectric slaves. Whichever method you choose, getting your flash off camera gives you a chance to dramatically improve the quality of your lighting.
This idea can work even better with a free standing monopod. This version has some kind of feet that extend to permit the monopod to stand by itself (The Trek Tech Go! Pro is a good example. Read my review of it here.) While not steady enough to do the work of a tripod, these units will be handy for use as an ersatz light stand.
When shopping for a monopod you normally have to worry about getting one that's sturdy enough to support your camera and biggest lens. For pros working with big telephotos and heavy pro cameras, this calls for a heavy duty, high end unit. Many of us also carry a smaller, lighter weight option for using as described above. Amateurs who don't work with such heavy gear can probably get by with a less rugged monopod.