What photography kit should I purchase?

I'm going travelling around Europe for a fair while this summer, and currently have a Nikon D90 with a 18-105mm lens and a tripod. I've been taking photos for just shy of a year now and have taken some really great shots and love experimenting with all the different components, but I know very little about the extra options out there, for example filter and other lenses etc. Does anyone have any tips on what to buy and why?

A polarizing filter is a very common accessory. It can be very handy if you are taking shots during the midday hours. Practice with it before your trip, though.

That is an excellent lens for European travel. It has all of the most used focal lengths. Take the tripod if you have room.