MapsForge

http://wiki.openstreetmap.org/wiki/Mapsforge: mapsforge provides a free, open-source, offline vector map library (and more) for Android. With the easy to use API you can build your own map application with less than 30 lines of code. Overlays (POIs and arbitrary polygons) are also supported.

Project page: http://code.google.com/p/mapsforge/

Fetching the source: git clone https://code.google.com/p/mapsforge/ I then File->Import into Eclipse two projects, Samples and AdvancedMapViewer.