XMPP

XMPP is the Jabber protocol. It is also used for GoogleTalk

See here for official documentation or here for the Wikipedia page.

I am interested in it for use with FoxtrotGPS and IncidentView.

I want to pass specific bits of data along from a server to a laptop in the field.

For FoxtrotGPS I want to be able to show the locations of "friends". Aka "presence information" http://en.wikipedia.org/wiki/Presence_information

In IncidentView I want to be able to pass along the information from a 911 dispatch center to a rig.

Working with .Net

..because I am interested in libpurple for Windows (and pidgin runs on Windows)

Downloaded and unpacked source from pidgin site.

It is easy on Ubuntu 12.04, once you install the build dependencies.

apt-get build-dep pidgin
cd pidgin
./configure 
make
sudo make install

Here are the official instructions for Building Pidgin on Windows