I've had the Sigma 10mm F2.8 EX DC Fisheye lens in my kit for a while now and although you can't use it all the time, it's a great little lens in a lot of situations. From architecture to portraits (see my Rules Are Made To Be Broken blog post) and everything in-between. With a fast wide aperture of F2.8, it's a great lens for low light situations.
The 10mm Fisheye is designed for APS-C size sensor cameras. The Nikon version produces a diagonal angle of view of 180o (154o on Sigma and 167o on Canon) and a minimum focus distance of 13.5cm/5.3 inches. The lens is predictably soft at the edges between 2.8 and 3.5, but at F4 and beyond it performs much better than expected. The centre of the lens is pin sharp and the bokeh when shooting wide open is fantastic.
You can pick one of these lens's up on Ebay secondhand for around £300, and it really is worth it.