Yahoo has had this feature for months, and even Google Mobile has had it! But finally, Google has added real-time traffic to Google Maps. According to their blog:

For more than 30 major U.S. cities, you can now see up-to-date traffic conditions to help you plan your schedule and route. If you're in San Francisco, New York , Chicago, Dallas, or any of the other cities we now include, just click on the traffic button to show current traffic speeds directly on the map. If your route shows red, you're looking at a stop-and-go commute; yellow, you could be a little late for dinner; green, you've got smooth sailing.

Sweet! Note that this won't work everywhere, as I emphasized in bold above.