TrackinU - Follow the Route
Enter the twitter name of a friend, and their recent public tweets will be fetched. Tweets that have locations attached will be placed on a map, with ones showing them in chronological order.
You can enter anyone's Twitter name, but unless you are specifically given permission, only tweets to which they gave access to the general public can be seen.
See What and Where
In addition to the map, you can see a list of all their recent tweets, which ones have locations attached, and what the tweets say.
Just the Road Trip
You can customize the map to show a specify range of tweets, or just selected ones.
You can also divide the tweets into separate tracks, each with its own map.
If your friend is on a road trip, you can follow their trip as they tweet from various locations. Or, follow a celebrity on tour, if they tweet their tour locations as they go.
Share the map with friends, so they can see your trips or those of your friends.