Old Tamron 500mm f/8 SP BBAR MC
The tamron 500 is a true classic and one of the better cardiotropic (mirror) lenses made. It is somewhat of a cult classic and produces "donut" bokeh. It will work with a D90, but it will be manual metering only. I have used several of the old adaptall lenses and some of them are still wonderful today. Some Nikon bodies such as d200, d300, d7000 (I think) will meter with these older lenses. The adapters can be found on ebay of course, and I think KEH has them as well. Sometimes it is cheaper to buy a lens with the adapter attached on ebay.
More info on adaptall-2 http://www.adaptall-2.org/ click SP lenses on left side