The 50mm F1.8 is value for money, and for portraits (75mm equivalent on the D90) its perfect for portraits.
For general purpose lens, there are many options, 18-200 VRII or VRI, 18-105, 18-135.. It really depends on how much you want to spend.... You can always test out some of these zoom lenses with your D90 at the camera shop and see whats going to work for you..