Comet Hyakutake, shown in the left figure from 1996 (Ref), was one of the brightest comets in recent years. The image at the right shows a Hubble Space Telescope image of the nuclear region (more info). The entire image is 760 km across; the bright spot at the very center is thought to be a jet of material, with the nucleus lying obscured under this bright spot.

Here is a more extensive discussion of the Hubble Space Telescope observations of Hyakutake, and here are the orbit and present position of Hyakutake. Finally, here is a link to some movies (AVI and QuickTime format) of Hyakutake.

See also this link for more pictures.