Well yes we have covered the topic before if you want to go beyond 1/1 life size there are specialist Macro lens like the Canon MP-E 65mm that can go to 5 times life size but is tricky to handle without a tripod at the max range.. I am unsure if any other makers produce a lens like the Canon,  I have use a sigma acromatic add on filter that is quite good but my own experience with the x2 filters or higher  you screw on the front apart from the sigma are not very good and worked much better on non SLR digitals I have owned and in any case will not add very much further magnification perhaps 1.3  as they are not designed to work on an SLR they tend to do odd things to the DOF and introduce noise in my humble opinion..

Danny O'Byrne  http://www.digitalartzone.co.uk/

"All the technique in the world doesn't compensate for the inability to notice" Eliott Erwitt