Okay, The most expensive and most versatile would be a set of bellows, these have a variable adjustment but auto versions are very expensive and bulk up the system a bit. Light falloff at the film/sensor plane can be marked at longer bellows extensions.
Next, you can get a set of extension tubes, auto ones are best but will cost you about £100 or so. They fit between the lens and the camera body and alter the lens to media (sensor or film) distance which enables the lens to be brought into focus at a closer distance than would normally be possible. There is usually a fall off in light with their use due to the added distance between the lens and media, this increases proportionally with the amount of extension used.
You can also get supplementary lenses (in a filter thread) to fit on the front of the lens, these work like magnifying lenses and offer the cheapest and easiest option, but are prone to distortions and the image quality is usually reflected in the price you pay. there is no increase in exposure required for their use.
If you have another lens, there is an even cheaper option which is to buy a male to male filter adapter to allow the fitting of the other lens onto the front of the camera lens, a 50mm is commonly used to do this.
