I've been a fan of old glass, ever since I had an Olympus E-510, that I used with numerous adapters, back in 2009.
On the D90 I've used the Micro Nikkor 55mm f/2.8 Ai-s. A blisteringly sharp macro lens that is also great for posed portraits. As its manual focus, you can pretty much forget about using it for photographing the kids tearing round. My 50mm AF-D lens struggles to keep up with my nephews at times lol.
Due to its focal length, its probably not the best for skittish insects but, for still life its great. They can be picked up for around £150 on eBay.
At the weekend I managed to get a near mint copy of the Nikkor 28mm F/2.8 Ai-s for £140. It arrived yesterday and I had a quick play with it. From the few shots that I've taken, I am very impressed with it. I love the close focus correction feature. I think I'm going to enjoy experimenting with this for portraits.
As Bump mentions, the loss of metering on non professional models of Nikon DSLR bodies will take some getting used to and require test shots and histogram readings, unless you have a hand held meter. Having said that, I recently came across "Dandelion" chips specifically for non CPU Nikon lenses. The chips attach the the rear of the lens and are programmable so that you can use metering and TTL flash on models like the D90 and below. I'm seriously considering getting one for the 28mm Ai-s, as I can't see me affording a D7000 for quite some time.
If you're interested in these chips for your own lenses here is the link
http://www.leitax.com/OlympusOM-lens-for-Nikon-cameras.html. They also do adapters so that you can use a variety of lens brands on Nikon bodies and vice versus.