Not worth it for a commute (you will be doing the same route over and over, and before long, you won't need any help with navigation), but I have a Garmin Edge 800. The edge allows you to plot out a course in advance and it will give you turn by turn directions, or just give it a final address and it will find that, as well as being able to search for close by things like restaurants.

I use the Garmin on my fun bike as a bike computer, and if I am travelling to unfamiliar areas, I use it to help with navigation.