If I had the cash I'd upgrade my Canon 100mm f2.8 macro to the new L version with IS. Doubles up as an amazing portrait lens too. Relatively expensive though.
You could probably pick up a used Canon 50mm f2 macro cheap, or Canon 60mm f2.8 EF-S.
The Sigma 105 and Tamron 90 are good 3rd party lenses.
If budget is really tight then consider some screw-on filters that fit onto your kit lens. They can produce good results, especially if you're not bothered about producing large prints.
It really depends on your budget, like Tom suggests.