OruxMaps allows for topo navigation while offline, by being able to read in OziExplorer formatted maps.
I have made a first effort to get maps installed, but it is rather complicated with many steps getting map data into the right form to use, and having to work with very tempermental utility software along the way. If I ever get a good method down I'll post up. I was using topo data from
http://www.topodepot.com/
I look at this as more of a gee-whiz kind of app than functional. For now, the real production setup is a ruggedized tabletpc with 7.5' (1:24,000) topo data hooked to the 60csx.
The other highly useful app is
GeoBeagle, which allows similar offline use. Specifically, GPX imported files with user comments are usable within the app. I ponied up for a year's membership to geocaching.com, which allows you to create huge GPX "pocket queries" via the site's batch server... essentially allowing big chunks of up to 500 caches to be downloadable into a file, then imported into GeoBeagle. There is a another caching app highly recommended on the market that seemed to me more online, but supposedly has an offline mode.
I run these with a Cyanogen-modded ROM G1