Garmin and Magellen are the big dogs. Garmin is the biggest; everyone does Garmin protocol.
I use ExpertGPS for the GPS waypoint management, TopoFusion for its networking and track reduction, as well as imagery, and Topo for historical trip management and trip planning.
If you use any GIS software (or AutoCad, or I guess photoshop), that's what DeLorme is like. Multiple layers of data you can turn on and off. Very useful once you get used to it. There is a steeper learning curve, but that's because it does more; most programs just work with one layer of data at a time. I sometimes have 10 or more I'm working with. It does have it's weaknesses, like any of the earlier mentioned programs. When I want trip planning, delorme is fine. When I want to go on the road, I take a Garmin. Much better road updates in city/surburban areas, and even their rural areas are getting pretty good. DeLorme has made a lot of good efforts to drag stuff into alignment, but in some lower-use areas, roads can still be 300 feet off. (It's based off the US TIGER maps, visible in TopoFusion)
DeLorme also has a layer for map markups. Text, callouts, lines, geometric shapes, etc. Convert tracks to a draw-markup line and cut the map, put it on your GPS, and you can always view it, without using up a track slot to store it. It's always "on" (Which I suppose could be a problem if you want a track turned off.) The bonus there is an infinite number of tracks, with notes, all displayed on your GPS screen.
When I'm 4x4ing in the Grand Canyon, I always take a laptop and a bluetooth GPS to run Topo. Much bigger screen than any GPSr I've used, all my added roads, all my added notes.
• Draw layers are here: http://forum.delorme.com/viewtopic.php?t=6941
• Grand Canyon example is here: http://forum.delorme.com/viewtopic.php?t=9278
• Tucson Mtn Park (a simpler example with waypoints, tracks, and draw layer) is here: http://forum.delorme.com/viewtopic.php?t=15943