iPhone Simulator is a free and open source simple command line utility for launching an iPhone application in the simulator. This allows for nice things like automated testing of unit tests, etc without having to open XCode.
You can run the application by first building your application in xcode using the xcodebuild command from the command line. You can then run it in the simulator using the following as an example:
./iphonesim launch ~/tmp/yourproject/build/Debug.simulator/yourproject.app
You need to point to either Debug.simulator or Release.simulator based on which build type you built with.