http://piggo.com/2010/09/my-answer-to-which-macro-lens/
I wrote that a couple of years ago now, but still holds true today, and echoes some of the recommendations above.
As JJJ points out, the Tamron 90 doubles as an excellent astro lens. It has curved blades which help reduce diffraction spikes which I find distracting on lens (as opposed to telescope) astro images.