Easy Rider is the quintessential motorcycle road movie, with the added benefit of being a key example of the post-classical pre-blockbuster era of Hollywood. Peter Fonda shows that he's his father's son, Dennis Hopper made a big splash as a director and leading actor, and as a bonus it has Jack Nicholson before he became typecast as "crazy Jack Nicholson". Rather pessimistic, however.
The Motorcycle Diaries is a biopic based on the diary of med student Ernesto Guevara, describing the pan-Latin-American road trip he took with his friend Alberto Granado, showing how it molded him into "el Che", the Marxist revolutionary he is known as today. Mexican teen heartthrob turned indie film darling Gael Garcia Bernal performs up to his usual standards. You don't have to be a Marxist to enjoy it... but it would probably help.
A little more upbeat than those films is Long Way Round, a travelog mini-series I just started watching on DVD, about actor Charley Boorman and his more famous actor friend Ewan McGregor riding motorcycles from London to New York, by way of Asia. (There is a brief scooter appearance before the ride begins, when their cinematographer has to ride a scooter with a "learner" badge to his motorcycle test.) This is the first of several travelogs Boorman has done (including Long Way Down with McGregor, from Scotland to South Africa).
Any others you can recommend?
This is Ewan and Charley back on the BMWs riding from the northern tip of Scotland to the southern tip of South Africa, by way of Italy, north Africa, and east Africa. 
Awesome! I'd love to see that.
