http://www.sigmaphoto.com/lenses/lenses_all_details.asp?id=3253&navigator=5 It looks like this is a manual focus lens, but for macro that’s how you should focus anyway
http://www.sigmaphoto.com/lenses/lenses_all_details.asp?id=3283&navigator=5 Probably more highly regarded than the previous lens, plus it will autofocus if you want.
This is a Tokina, also a very good lens. The downside to Tokina is that everyone says they make crappy lens hoods.
http://www.tokinalens.com/products/tokina/afl-m100-a.html
And also two Tamron lenses
http://www.tamron.com/lenses/prod/90mm.asp
http://www.tamron.com/lenses/prod/180mm.asp
It would be hard to go wrong with any of these lenses.