Boston Marathon, Paris International Marathon, Big Sur International Marathon (i might be a little biased about Big Sur because I'm running it!), the London Marathon, the Madrid Marathon, and a slew of others that definitely deserve a mention, check out the US/Canada Marathon Guide Calendar and the International Marathon Guide Calendar.
Part of what gets me psyched about running a race is the history behind it and/or the historical significance of that particular race. I get a similar feeling when I go to a rock concert or music festival. Here are some old articles I dug up about some of these marathons. They're pretty cool...
1) Madrid Marathon Re-cap April 1985
2) London Marathon Re-Cap March 1981 (Dick Beardsley)
3) American Wins first Olympic Marathon on American Soil 1904 (St. Louis)
4) Tough Time For Americans in 1900 Paris Marathon
5) "Famous Indian Runner wins Boston" Tom Longboat April 1907