See the tutorials directory for a progression of example programs all the way from hello_world in the 01_basics sub-directory, through to more advanced vision and action examples.
See the apps directory for some fun fully-featured programs like a remote control and a desk security guard.
See the if_this_then_that directory for some examples that connect Cozmo to the "If This Then That" service.
See the multi_robot directory for an example of how to connect the SDK to multiple devices and robots.