This is a case of self discipline saving you the most money. Don't rely on technology
to do what you should be in the habit of doing in the first place, because you're talking about spending $15-30 on a chunk of technology that replaces the function of you simply operating a switch which only takes a few neurons and a fraction of a couple calories to do.
The self discipline "muscle" can be the strongest and most beneficial muscle you can exercise, but you have to develop it like any other muscle. Learning to use it will be the best thing you can do to save money across the board.
Now, could using a technology solution theoretically, potentially save you that much in wasted electricity by doing it for your forgetful self? Depends, possibly, given enough time. If it's an incandescent, probably pay off after six months of
continuous on time figuring 15¢/kWh. If it's an LED, it could take years to break even. (CFLs are not recommended for short-on lighting tasks in closets and pantries for example, it shortens life drastically.)
If you're still convinced that technology is the solution:
http://www.homedepot.com/b/Electrical-Dimmers-Switches-Outlets-Motion-Sensors/N-5yc1vZc32r