In addition to the many great programs listed here (particularly the electronic book and manual readers) I want to add some that I use on my EVO 4G and G1 which use the Android OS.

There is a great offline GPS mapping program available for Androids (and Windows Mobile smartphones) known as BackCountry Navigator. You can download USGS or MyTopo maps (or others worldwide) for use even when you don't have a data connection, or even GPS satellites. It does tracking.

Besides MapDroid (mentioned earlier) there are other similar ones such as Maverick and Orux Maps, but I like BackCountry Navigator the best.

I don't remember anyone mentioning the Latitude feature of Google maps which enables you to know location of you and others on the map which can be useful in meeting up. There are other programs that do this also. Just keep OpSec in mind.

There is another program that is neat called Layar. You hold your phone up and pan around and it shows you points of interest showing the direction and distance superimposed on what your camera sees. Great for locating gas stations, ATMs, restaurants, etc.

Google Goggles could help identify unknown terrain features, landmarks or even people (I have used it to identify avatar photos).

Programs like Qik or videoconferencing could broadcast what the phones camera is seeing and thus be used as a security camera.

There are programs that allow the camera LED flash to be used as a flashlight.

For anyone who cannot identify the North Star (LOL), there is Google Sky Maps.

If wimax/4G or cellular wireless is (still) up or within range, the EVO can be used as an access point for computers to access there internet.