Something new I learnt with my new flash gun recently - when you set your built-in flash in "Commander TTL" mode, you now possess the capability to project two concurrent light sources on your subject - one from the off-camera wireless SB-900 and the other from the built-in speedlight!
With this combination, you may be able to pull off some simple macro shots quite nicely without any additional equipment. On a side note, if you would like to tone down that built-in speedlight of the Nikon D90, you simply need to switch it to the "Commander --" mode. This way, the flash effect will primarily come from your Nikon SB-900. Its that simple. So much for my version of flash 101 photography.
With my trial also covering indoor photo taking, the grim, hard fact I found is that you will still need at least three proper speedlights to light up a decent indoor area. That's the reason why some pros have about twenty of them! Goodness!