If you do not own a SB 600 or 800 flash it is a must have. You will not be sorry. Great for indoor and out and has wireless connection to onboard flash.
A set of Alien Bee strobes...
A 100mm macro lens...
A good bogen tripod and head...
the aforementioned flash...
Tamron 28-75 2.8 lens...
a 5-in-1 refelctor...
Cokin filters or other filters...