An historical reference, along with a solution and interesting links for the Konigsberg bridge problem is found at this site:
This site from Math 103 at the University of Alabama provides a nice overview of Euler circuits and Hamilton cycles. Click on the topic in the left column.
This site provides a thorough overview of the Traveling Salesperson Problem (TSP):
This site provides a TSP problem solver:
This site, by Chris Caldwell provides Graph Theory tutorials:
An interesting site about Mobius strips is found at this site: