Here is another demonstration. This image was taken with my K10 and not edited at all; straight out of the camera.

The lens is a SMC Pentax FA 28-90mm mounted in reverse. Very simple alternate to having a macro lens.
Remove the lens from your slr, turn it around so the "face" of the lens meets the body at the lens mount and get closer to the subject until it comes into focus.
It takes a lot of light because you are going to want to close the aperture down by at least half and the lens of course needs to have an aperture ring.
This image was shot at f19 at 0.7 seconds; tripod mount obviously.
Got an old manual lens with a short profile? They work the best I think. Reverse a 50mm and it is better than one to one magnification if I am not mistaken with a focus distance of about six inches maybe.
Extension tubes and plenty of other tools work well of course but it is still fun sometimes to just remove the lens and either hold it with your hand to the camera body or get a reverse ring adapter.

This may be nothing new to some of you but I get a kick out of it everytime. I have manual 50mm, an AF 35-80mm and an AF 28-90mm that I keep around almost specifically because they do so great reversed. Almost more fun reversed than proper mount.