The SB-800 was discontinued about a year ago and was replaced by the SB-900.
The SB-800 is still available as a used item though.
Compared to the SB-600 the SB-800 has more power, can zoom to 105 mm, has a built-in bounced flash fill card, and has an accessory that for a 5th battery to shorten the flash recycle time.
The SB-800 has a Commander mode, so it can use Nikon's CLS to control other Nikon SB-600, 800, and 900 speedlights. The SB-800 also has SU-4 mode that lets you trigger it with the flash from another speedlight or the built-in pop-up unit on the camera.
The SB-800 also has a jack for an external power source, a port for a PC sync cable (PC type connector, not a computer) and a port terminal for multiple TTl speedlight operation.
The SB-600 can only zoom to 85mm and has none of the SB-800 features, ports or terminals listed above, which is why it costs less.
I happen to have a spare SB-800 for sale (only $325):
http://www.thephotoforum.com/forum/buy-sell/222900-fs-nikon-sb-800-speedlight.html if you were to be interested.