Usability 
Traffic.com has a nice new mobile web version of their real-time traffic alert system at mobi.traffic.com. It currently covers 49 US metros. You pick your city from a drop down and then choose a highway from a paged list or click "Hotspots" to see the 10 most congested roads in your metro. Traffic.com uses a nifty and intuitive thermometer graphic to show the relative congestion on each road. You can drill down into a particular highway and see average and slowest speed, delay in minutes and incident reports.
This is the first good, free US mobile traffic site I've seen. The only usability flaw is that the city drop down includes all 49 cities in a long scrolling list. For better mobile usability the city picker should really be divided by groups, (A-F, G-K, etc) of ten or less cities per group. You can avoid the drop down and generally make Trafic.com mobile much more usable by registering on the PC site and setting up "Your Trips" where you can define turn by turn routes. Then when you log in to the mobile site you can go to your trips and see the current conditions of each segment of your trip.