the MP-E-65
This is a lens that really has no equal anywhere and is the reason I want Canon body even though I shoot Nikon. Around 5:1 magnification, but has a really steep learning curve. Focusing is extreamly tight and DOF can be a bear also. Most of the guys that use them use rails. Off camera flashes are also common with them.
I do quite a bit of my stuff with off camera flashes. When getting that tight up against something, light gets tough to come by. Most on camera flashes are too far away when shooting something a half inch from the lens. I'll get you some pics tonight when I get home.