MapQuest, which has been quite busy of late rolling out desktop site improvements and a new Local product, has just launched a new mobile site optimized for the iPhone. It features maps, directions, business search and gas price information.
Overall it offers a very strong user experience, nicely rendered. The only drawback is that because it's Safari-based and not an application it can't take advantage of the phone's built-in location awareness. Consequently most locations need to be manually entered.