* Sigma 50mm ($250)
* Quandary 50mm (made by Sigma and about $150)
* Nikon 60mm 2.8D ($330)
* Tamron 90mm ($400)
* Nikon 105mm 2.8D ($500 used)
* Nikon 105mm 2.8D VR (~$750)
* Sigma 105mm
* Tokina 100mm
Out of that list: Nikon 105 VR > Tamron 90mm > Nikon 60mm > Nikon 105mm > Sigma 105mm
That's how they stood when I tested them against each other in terms of sharpness, contrast, build quality, etc.
Of those, I still own the Nikon 105mm VR, Tamron 90mm Adaptall mount, and the Nikon 60mm.
Also, if you're really in a bind for a cheap macro lens, take a look at the 50mm f/1.8D along w/either a reversing ring or a set of Kenko tubes. It'll cost less than the cheapest macro above (other than the Quandaray, which isn't really made by Sigma; last time I checked, they're made by Ritz/Wolf Camera) and will give you a fast 50mm prime.
Lastly, let me know if you want to check out any of the lenses above at the next BAR meet.