Need somewhere to store your files but have limited storage space? This inexpensive project tucks away those files in IKEA's popular cubby. Instructables user acoens offers the step-by-step instructions. Besides the IKEA Expedit cubby and the Expedit single drawer insert, you'll need to metal rods and two L-shaped angle brackets, along with some standard tools.
Although Expedit is now discontinued, you can do the same with the new Kallax system. The whole project takes about an hour, and then you can hide your files in plain sight.
IKEA Expedit Filing Cabinet Hack [Instructables]