Are you photographing stuff that sits still or things like insects that need a quick focus ?

Because my budget is tight and I cant afford a 100mm AF Macro for my static macro's of water drops (where I can focus once and then take pic's all night from a tripod) I use a 55mm M42 (Pentax screw fit needs a canon adaptor) lens with 1 small macro extender. Gets pretty close.

http://www.flickr.com/photos/drippy2009/sets/72157615864499805/
Most of the shots in here were taken with that or a tamron 70-300mm

Or if you want REALLY close add a full set of 3 macro extenders
http://www.flickr.com/photos/drippy2009/sets/72157616048839358/

Depends on what your shooting and your budget really