This isn’t really a post, just a collection of links I use with my Garmin.

GNU mapping software for Linux
http://www.qlandkarte.org/

Garmin Basecamp – I’ve been using this quite a bit on OS X.
http://www8.garmin.com/support/download_details.jsp?id=4435

Generate street maps for Garmin GPS devices. (worldwide)
http://garmin.openstreetmap.nl/

Agency land maps KMZ/KML – based on PAD-US data (BLM, Fish and Wildlife, defense, USFS, exc…)
http://apeoid.com/kmz-kml-padus/

PAD-US database is a work in progress of spatial data intended to facilitate conservation and public land protection, as well as bioinformatic study.
http://gapanalysis.usgs.gov/padus/