This isn’t really a post, just a collection of links I use with my Garmin.
GNU mapping software for Linux
Garmin Basecamp – I’ve been using this quite a bit on OS X.
Generate street maps for Garmin GPS devices. (worldwide)
Agency land maps KMZ/KML – based on PAD-US data (BLM, Fish and Wildlife, defense, USFS, exc…)
PAD-US database is a work in progress of spatial data intended to facilitate conservation and public land protection, as well as bioinformatic study.