GPICommander is two things: a USB to GPI Converter and a software. You can send or receive GPI triggers using AppleScript, so it can be used with any application under MacOS X. If needed, an SDK is available if you want to have direct access to it without AppleScript (it also runs on Windows then and it enables you to control the unit by writing C code.)

It has 24 inputs (open-collector type, common ground) and 8 independant relay outputs but since each unit has a unique serial ID, you can chain more than once on the USB bus to have more inputs and/or outputs.