We here at NovemberKiloEcho use open source software (OSS) to make our robots move. Of course we contribute back to the community, too. Until we publish our gits, please see the following links as some of the awesome projects we use and you should too. 
Robotic Operating System (ROS)
We use Willow Garage’s awesome ROS code to make our bots move, plan, and some soon-to-be-announced very cool things.
Gazebo 3D Simulator
Willow Garage fanboys like we are, we prototype all of our functions and capabilities using Gazebo. It’s not perfect, but the folks at WG (via the newly created Open Source Robotics Foundation) will be providing a super-updated version of Gazebo for the participants of the next DARPA Robotics Challenge (Humanoid Robot) of which we are a contestant (more on that if we get selected to participate).
OpenCV
OpenCV is a key component for all vision components on our robots.
Point Cloud Library
Ok, enough Willow Garage drama. Errr… Ok, also check out their Point Cloud Library (PCL) as a very nice library for environment modeling (synthetic vision) and object detection.
FreeIMU
Fabio Varesano has been working on his FreeIMU library (he has his own hardware too) for quite some time. (I feel like I’ve read every post he’s made in every community that is relevant as he was creating it). Guess what, It rocks. V 0.4 now supports Sparkfun IMUs directly. This library provides everything from RAW 3D sensor data to very efficient Quaternion data (google it!). Of course, we’ve developed our own IMU software, but it’s not quite ready for the show, and it will be quite some time before ours stands above Fabio’s work. Go get it here.
ARToolkit
Sure it’s not as powerful as its commercial (licensed) brother(sister?), but great things can and have been done with ARToolkit running behind the scene. Look at Hunter as a prime example.