Extension tubes are a cheap way to start macro photography. They are basically hollow tubes with electrical contacts. The tubes comes in a variety of lengths: I have a set of three -- 13 mil, 21 mil and 31 mil. They can be used individually or in combination.

As I recall, the magnification is a function of the ratio between length of tube and focal length of the lens; so on a 50mm lens using my 21mm and 31mm tubes together would give just over 1x magnification.

The downside is that work distance is pretty small, so they don't work too well with anything alive.