Offline map browsing with large OSM datasets (e.g. all of europe)
directu is a free iOS app offering offline map browsing with OpenStreetMap data bundled into the app. In fact it is a set of apps with separate listings on the appstore for several regions of the world and for iPhone and iPad.
Map data is bundled with the app, so this takes a while to download from the appstore, and doesn't get updated with changes from the OpenStreetMap community until they push out an app update (Last update April 2011)