I would recommend the AF-S 60 f2.8 or the AF-S VR 105 f2.8
Always depending which focal length you need. If you also want to take portraits the AF-S 60 is a excellent choice for a crop-camera. There is no mode which has to be changed. You can use the lenses for closeups and macros as well as for portraits.
I use both lenses on DX and FX cameras and would never change them.
But if you want to take closeups of insects - of course the 60mm (90mm on your D90) are quite short.
This image was shot with the AF-S Micro 60 f2.8 on a D3
.. and this was done with the AF-S VR Micro 105 f2.8 on a D70
But you will not regret you choice wether it is a AF-S60 or 105.
tripods, camera support from ReallyRightStuff and Jobu design.
I should suggest the 105mm (if budget is not an issue of course). There are a couple of good reasons for this.
Consider that the longer focal length allows you to keep a larger distance between you and your subject; non only useful for live subjects but also for shiny inanimate subjects too. The longer distance from your subjects could give you more control over unwanted reflections.
Another reason is that the 105mm is a good portrait lens too.
The 60 mm micro lens is excellent, the problem is to be attached to the objects you photograph! Everything becomes difficult and impractical! Get the 105 (budget permitting), just spending once and you will not regret!
im surprised noone here suggested any third party lenses! the tokina 100mm is allegedly better than the nikon 105 and half the price but because of this its never available, but, the tamron 90 is about equal in sharpness to the 105 in the tests I have read, and it is also less than 500 bucks. it will give you the focal length you need without breaking the bank. It doesnt have vibration reduction, but neither does the 60mm. I just got one and it has taken my photography to a new level.