libusb is a C library that gives applications easy access to USB devices. Many of my previous projects involving USB devices (e.g. the EZ-USB breakout board) use the libusb to communicate with it from the PC.
Since libusb is implemented in (plain) C, its headers had to be translated to be used with Pascal. I've also added an object-oriented wrapper. Please find the source code at https://github.com/hansiglaser/pas-libusb.