iPhone: OffMaps downloads regional maps for offline map access on your iPhone, great for finding your way when you don't have a data connection or avoiding data roaming charges if you're travelling abroad.
OffMaps lets you download "base maps" of a city, and then you can choose to download particular areas in more specific detail. The maps don't require a connection of any kind, which is great if you don't have Wi-Fi and/or GPRS/3G access — particularly useful if you're travelling abroad and don't want to incur high data roaming charges or you don't have a data connection and still want to find your way around. And since your iPhone's GPS location works without access to a data network, you can still find where you are on the map without a data connection.
The maps are often huge downloads (they can vary from 100MB to 200 MB per base map), so it's best to download the maps when you have some time to set aside. The app also lets you access Wikipedia entries offline, and includes two city guides of your choice through two "free" tokens. OffMaps has an extensive array of locations, including every US county and thousands of international destinations. To get a sense of how each guide is covered in the iPhone app, the OffMaps website offers a map for seeing if your location is well covered.
OffMaps is available in the iTunes App Store for $2.49, and comes with two tokens. Tokens are required to download city guides, and additional tokens can be purchased.