Rating: 5 / 5
With Maplets I easily download a wide variety of maps, such as NPS and USFS maps, local/state/county park maps, transit system maps, town maps, airport maps, cycleways, etc. Many of these are geographically calibrated, so that your location appears on the map, others (such as transit maps) just provide an offline copy of the paper map.
I've been using this app for the local and national parks in the SF Bay Area - really useful because the park maps show trails that are not on the USGS maps. The app does not have any features other than showing your current position on the map, so it is not a replacement for a fully-featured map/gps app. It is extremely easy to use, nice simple interface to find and download maps. I love this app.
The inventory of park maps for the US is very thorough, less so for the rest of the world. If you know of a map that should be included submit it and the developer will work to add it; they added all the maps from WalkingCoastalSydney.com.au within a few days of my request!
Map Sources: Huge variety of maps. See mobilemaplets.com/places
Additional charges: None. All maps included in original purchase price.
NOTE: Here is a list of all the apps I've evaluated..