Quartz Composer is a free application from Apple which allows you to render graphics using a visual programming language. You can get it by installing the Xcode suite from your Snow Leopard install CD, or you can download the Xcode suite here (free to Apple Developer members, $5 otherwise). Since Quartz Composer is only for Macs, the information on this page only applies to Mac users.
Here’s a video overview showing how to use Synapse to make visuals in Quartz Composer. You may want to view it in full-screen or on YouTube for higher resolution.
The “Synapse Kinect” plugin, found on the Download page, allows you to get the depth image from Synapse into Quartz Composer.
If you want to receive joint positions and events in Quartz, they are sent via OSC. Quartz has a built-in OSC Receiver object, but I recommend using qcOSC as it tends to be overall more reliable. It can be found on the Download page.
For some reason, qcOSC can’t receive messages sent directly from the Synapse app, it causes a crash. For that reason, I use a Max application as a passthrough to send the OSC messages to Quartz. Run the “quartz_passthrough” application included in the example Quartz project, and set the qcOSC object that you drop into your Quartz project to listen to port 12348. When running my example application, however, you should run “quartz_passthrough_plus” to get all the extra bells and whistles.