Dr. Ron Blakely of Northern Arizona University has a number of beautifully lavish maps of the paleogeography and the geologic evolution of North America. These are colourful and clear and the most detailed of any such maps freely available. The 41 maps are available in jpeg format and illustrate the appearance of North America over the past 500 million years. Outlines of state, provincial and national boundaries provide a useful reference point for each map.

Also available on his site are 20 or so global snapshots of the world over the past 600 million years. These are available in Mollewide, orthographic and geographic projections.

By way of BLDGBLOG who has a nice narrative to go along with the geologic evolution of North America.

Update: PerryGeo picked up on this post and turned the images into an animated sequence. Now the entire geological history of North America is available in a 30 second animated gif!