if by "stimulus" you mean you want to generate test vectors to excercise your device core, then may be you should look into verisity eVEC tool. It is great for generating random and controlled test stimulus. if, one the other hand, you want to develop code such as device driver, then look into NuMega's driver studio. This tool writes your device usb driver for you. all you have to enter is a few question about your endpoints characteristics along with some other vendor information. if you want to write the driver or the test vector yourself, then search the web for "linux usb driver development" there are lots of good primer and code templates out there to get you pointed in the right direction.