Wikimedia
Wikipedia: Seven Bridges of Konigsberg
This Wikipedia site introduces the seven bridges of Konigsberg problem, a classic graph theory problem solved by the mathematician Euler in the 18th century.
Wikimedia
Wikipedia: Graph Theory
Wikipedia provides an introduction and overview of graph theory. In addition to introducing the basic terms, there are related links to classic problems in graph theory such as the traveling salesman problem and Dijkstra's algorithm.
Other
Los Alamos National Laboratory: Games on Graphs
Learn to understand graphs and graph theory by playing games from Los Alamos National Laboratory. The site also provides definitions and related links.