Ultra-compact Quantaray 500mm f/8 mirror lens (see photo). This lens uses a folded optical path and is therefore extremely compact -- a half-meter focal length in a lens less than 4 inches long. (Think of it as a tiny Schmidt-Cassegrain telescope with a 2.5-inch aperture.) It comes with a Nikon mount, but because it uses a standard T-mount thread you can remove the Nikon mount and replace it with a mount collar for any kind of camera. Accepts 72mm front filters or 30.5mm rear-mount filters. Includes two neutral-density filters (ND 2x and ND 4x).
One nice feature of this lens is that it can focus very close -- down to 5-1/2 feet. With a 500mm focal length at that distance you can take great macro shots from several feet away -- very handy for flowers, insects, etc. (especially insects that don't like to be approached or that you don't _want_ to approach~).
In like-new condition (used only for some optical experimenting, never in the field), $75 + shipping. PayPal preferred.