roslaunch external program

edit

Nate, the suggested method was to wrap a system call.

See this node:
http://www.ros.org/wiki/webots_run
for an example of running an executable from a ROS package.

Two useful behaviours are:
1. This node launches with required="true" and the source code doesn't ros::spin. The result is that if you launched everything from one launch file, when you close the simulator, it will drop back to console. (this is good once the system is complete)
2. Alternatively, you can start roscore in 1 terminal, this node to launch the simulator in a 2nd terminal, then in the 3rd terminal you can keep re-launching your controller nodes while you debug them, without taking down the simulator front-end each time.

Refs:
http://answers.ros.org/question/33772/run-shell-command-from-launch-script/
http://answers.ros.org/question/51474/can-i-run-a-bash-script-using-roslaunch/

p.s. Any idea how long before ROS Fuerte users will be able to seamlessly swap to Gazebo stand-alone? ;-) (eg. if currently using the PR2 Controller Manager joint controllers)

You could use the aliencontrol ROS package.

This package contains a node that starts the external application. Almost more importantly, the node also takes care of a clean shutdown, no matter whether the external application or the containing ROS node is terminated.

It is run like so (if you wanted to start top):

rosrun aliencontrol aliencontrol top

If you want to embed it in your launch script, add:

<node pkg="aliencontrol" type="aliencontrol" name="aliencontrol_top">
    <param name="cmd" value="top"/>
</node>