Via AppShopper, I learned that the 'You Need A Map' app for iOS will be free for the month of April.
From the description: "A map that covers all 50 states. [...] It contains every mountain, hill, stream, lake, pond, freeway, highway, local road and track from two large online databases compressed into a 2GB app. In addition it contains many buildings, trails, railways, points of interest and polygon features. [...] Two maps in one! You Need A Map contains the roads from OpenStreetMap.org AND the US Census. OpenStreetMap has more accurate highways while the US Census has more accurate local streets. [...] You Need A Map supports GPS tracking while the app is in the background."