What lens should I purchase next for my D90? (Please see details)?

I'm slowly building up my gear and am looking to purchase a new lens within the next few months. Should I go for a wide angle lens? Another prime? A nice zoom?

This is what I have gear-wise at this point:
-Nikon D90
-18-105mm kit lens
-50mm 1.8 prime lens
-sb600 external flash
-gary fong lightsphere (cloud version)
-wireless remote
-tripod

I mainly do portraiture type photography, although don't have any type of studio yet. Most of my work is done outdoors although I do some indoor as well.

I'm looking for a third lens to make my collection a little more well rounded. I'm taking photos of couples, families, and also solo portraits.

Should I go for a wide angle lens? Another prime? A nice zoom?

For portraiture? You don't need another lens, unless you want a backup. A wide angle lens will distort the face. Get this if you're doing group head and shoulders shots. But don't go too wide, because of the distortion. Your backup portrait lens could be a large aperture zoom around the range of your 50mm, say a 35-70 f/2.8 or something similar.

The classic portrait lens is an 85mm on a 35mm film/sensor. Your D90 has an APS-C size sensor, which makes 50mm ideal.

I'll agree with joedlh, you don't need another lens - if you would, then you'd know what to use it for and what kind of lens to get. If you just get lenses out of a diffuse feeling that it may be time to get another lens, then you'll be wasting your money.

If you do have some money to spare, then consider getting a second flash, light stand and softbox or umbrella. That'll probably serve you much better for your kind of photography than another lens would.

You are already well equipped for portraiture and studio work. If you feel frustrated by the lack of extreme wide-angle on the 18mm then indeed get a wide angle, but take it slowly, maybe try to borrow an extreme wide-angle to see what it does. For portraits maybe get a further external flash, rather than spending quite a lot of money on a good extreme wide.

35mm f/1.8

although a macro lens might lend itself to something a little more radical with your subjects…

It seems like you should save your money for a nicer body, professional prints/portfolio of your work, or for classes on how to used your gear better. No offense on the class thing but we can all always learn more and knowledge is often more useful than another lens when one is at the stage you appear to be at. Sometimes we can get obsessed with buying more gear when we could have spent the money on more basic but crucial things.

These are the lenses that you can have:-
DX AF Nikkor lenses: All functions supported.
Type G or D AF Nikkor: All functions supported (PC Micro-Nikkor does not support some functions), IX Nikkor lenses not supported.
Other AF Nikkor: All functions supported except 3D color matrix metering II. Lenses fo F3AF not supported.
AI-P Nikkor: All functions supported except 3D color matrix metering II
Non-CPU: Autofocusnot supported. Can be used in exposure mode M, but exposure meter does not function. Electronic rangefinder can be used if lens has a maximum aperture of f/5.6 or faster.