Kenko Extension Tubes
There's no glass involved. These tubes are attached between the camera body and almost any lens (not compatible with Canon's EF-S lenses) to shift the available focusing distance. The effect is almost exactly like that of a macro lens, allowing closeup shots that weren't possible before, with an extremely narrow depth-of-field.
You can use all three tubes together, as seen here, or one at a time, or two at a time. The Kenko set that I bought includes tubes in three sizes: 12mm, 20mm, and 36mm, for varying levels of intense macro capability.
(This shot taken with my Canon EF 50mm f/1.8 II lens, the closest thing I had to a macro lens before buying the tubes.)