It's still in Beta form, but Avenza maps will soon be available for Windows phones, giving those users the ability to download and display georeferenced PDF maps, and to use the maps for navigation. Among other uses, this utility for iPhones has become ubiquitous on wildland fire incidents, since this free app allows firefighters to navigate using daily Incident Action Plan maps, complete with USGS topo basemap, fire perimeter, assignment breaks, and other important fire lines/points.
I'm sure you can think of many other uses for offline, georeferenced, custom maps on your phone. The Beta version is already available to the public. If you are interested in becoming a tester, and providing input on the look, feel, and performance of the app, you can apply here: