Eurail

I bought a Eurail Pass for my trip to Germany. Eurail can be an extremely good deal if you use it correctly. I bought a six day Denmark/Germany pass, which allows me to travel by train in those two countries for six days in a two month period. The six days do not have to be consecutive and you can ride the train as far as you want, getting on and off as often as you want. It was something like $320, which means each day is worth $53. To put this perspective, a single-use DB ticket costing $50 will allow you to travel across approximately 1/6 of Germany depending on speed of the trains.

So you can see how this is a good deal. In only one of the six days you can get your money's worth by traveling across Denmark and most of Germany, which is basically what I did going from København to Köln. Never forget the downside, though: it's a train... slow, tedious, and often filled with annoying people (a.k.a. children).