The unstoppable Google empire has added real-time traffic data to Google Maps. Currently the traffic data is available in 30 U.S. Just click on the Traffic button and see current traffic conditions visually represented on the map as green (no traffic), orange (some traffic), and red (heavy traffic).

Unlike my portable GPS that supports real-time traffic, can route around potential traffic problems, and factors traffic into the estimated arrival time, Google Maps currently just displays the traffic conditions - it doesn't change routing or arrival times based on that data (yet). No one seems sure of where Google is pulling the live traffic data from. Looks like they may be aggregating several sources into one view.