Well, I hate to disappoint, but the site I use most doesn't have a way (
that I know about) to download things to a GPS.
What I like about it is that I can plot points on the map and then pass the marked map as a link in an email or forum post. For example, here is
a trip in the San Gorgoino Wilderness that I'm planning for this Spring.
I use this for planning with friends who will be coming and also
to file a trip plan with my wife with whom I check in with after the hike. One time, I was seriously delayed on a hike. My wife called SAR which was totally embarrassing since a) I am NOT a no-nothing "tourist" hiker and b) I was in no trouble but had merely encountered a very washed out old trail that took a lot longer to negotiate and had mis-estimated the time required.
I was completely apologetic when SAR met me a few miles from the trailhead. They told me, "no, no, don't apologize. Your wife sent us your marked topo map, and you were
exactly where you said you'd be. This is the easiest 'rescue' we've ever done."
So, embarrassing story, but maybe a good object lesson. Having an marked topo map that is
emailable could really speed up the reporting which in turn might speed up the search.
HJ