Two of the most interesting and useful mobile sites that I noticed are Traffic.com and Trafficland.com. Both of them will allow you to monitor traffic for 30 or so metropolitan areas across the US.
Traffic.com displays city maps with live traffic cameras. They also promises to send you free traffic alerts by e-mail, phone, text message and RSS feed.
Trafficland.com works similarly, but they have expanded their free services outside of the US in cities in Australia, Canada, Denmark, New Zealand, and the UK. For a fee Trafficland also gives you the ability to store frequently traveled routes, view them before heading out the door and "access live traffic camera views from web-enabled cell phones and PDAs using TrafficLand’s AirVideo™ service."
Very handy and very convenient. I live in NYC, so I plan to use both web pages heavily.