Google Street View, Houston Circa 1891 and 1912
We seem to be all about the zooming in on graphics today.
Milam & Pease: Current home of the Houston Press
A site called BigMapBlog apparently specializes in adapting old bird's-eye map illustrations of cities into Google Street View-like zoomability. They've got Fort Worth in 1891 and Amarillo in 1912, and they've got Houston for both years.
Hop on downtown, and after a leisurely trip through the countryside, you're in the downtown of the other city.
Many of the street names on the maps no longer exist, so it can be difficult to locate modern equivalents, but it's still fun to try out.