Another alternative to extension tubes are diopter "close-up" lenses. These are lenses that screw into the filter mount on your regular lens. You'll lose infinity focus but none of the automation. The downsize is that if your lenses have different filter sizes you either need to buy a set for each lens or step-up/down adapter rings. The effect on image quality depends a lot on the quality of lenses.
http://www.ulrix.de/misc/closeup/en/closeup.html
http://www.ulrix.de/misc/closeup/en/closeup.html
