I'm shopping around for a handheld GPS, and from what I'm seeing, none of the big vendors (Garmin, Magellan and Delorme) make a unit that can use, for example, a simple georeferenced TIFF or a Shapefile as a background layer on screen. They all want you to buy (or subscribe to) their own closed-off product which basically just repackages data that is often otherwise freely available (like USGS quads) and allows you to use it with their GPS units.
Is there a vendor that sells a device like a Garmin GPSMap or Delorme PN which can use freely-available GIS data standards like TIF, SHP, OSM data, and so on? I know Trimble's handhelds can do some of this, but they are much more expensive.
Best Answer
Depending on your level of accuracy I would suggest the following:
Low level Accuracy
Use QGIS for Android (free and has a gps toolbar) along with your tablet or handheld native gps
High Level Accuracy
Use QGIS for Android along with your vendor gps