I was looking at an area with a lot of trails but no PDF/JPG or any kind of trail map that I could find anywhere to use to make a KMZ (custom map). I was able to find the trails I wanted on openstreetmap.org, so I started poking around to see if there was a way to get those onto my GPSr. The solution was amazing:
1) go to
http://www.osmmaps.com/2) download the IMG / zip file - took 3 hours, so I let it run overnight
3) Unzip the IMG file - for me, the US file was / is 3.5 GB (yes, gigabytes)
4) rename the IMG file (since my Garmin GPS supports multiple IMG files)
5) copy the IMG file to the "\garmin" folder on the SD card in my GPS
6) "Enable" the OSM map on the GPS
7) At this point, I couldn't see the trails on my GPSr that I could see on the openstreetmap.org website. I figured I was out of luck...but...

So this step may only apply to my GPS (GPSMAP 62/78 series), but perhaps there are similar steps for other Garmin GPSr: go to SETUP MAP, ADVANCED MAP SETUP, DETAIL, select MOST, and bingo! I now have the entire US version of openstreetmap.org on my GPSr, including all the trails you can see on that website. Absolutely amazing.
Caveats:
This IMG file is not transparent, so I can't see any other IMG files at the same time. I could probably figure out how to fix that, but I haven't yet.
This IMG file has no topo contours (and since it isn't transparent, I can't see the contours from my other IMG file, even with both of them enabled).
Openstreetmap.org doesn't have as much trail detail as some of the PDF/JPG trail maps that I have found, so I'm not going to give up on my KMZ collection. [But I was pleasantly relieved to see that most of the OSM trails match the location of the trails on my KMZ maps!] So as another source of trail information, it is great. And it was simple to get onto my GPSr.
Dave
my KMZ collection:
http://gpsfiledepot.com/maps/byuser/13384/