Windows Live Search Driving Directions. A very nice implementation of driving directions which remembers your most recent entries and offers you the option of viewing overview or turn by turn zoomable and scrollable maps with traffic overlays. There is a nice hybrid map option which combines aerial photos with street and highway overlays.
I really like the mobile versions of maps from Microsoft's MapPoint division which are very attractive and detailed and I think easier to read than most of the competitions.
Bing (formery Live) no longer offers mobile driving directions