The Sigma 105 is a cracker. You will need to get the flash somewhere near the end of the lens too. Or, you could just go for some extension tubes or a Raynox.
The Raynox is a very cheap way to get started with it, just got one myself and am more than happy with the results, a little more DOF would be nice but I cant grumble at the price - only £40
There's some pics on my flick, and also a big thread on this forum somewhere.