This is small versatile USB board developed by Brian Schmalz. He used PIC-USB capable microcontroller for easy USB interfacing and this way got simple parallel port replacement.
There is not much about circuit itself as there is only PIC microcontroller, USB connection and I/O pins. Such simple design allows making smal PCB that may fit anywhere starting from integrating in to test-boards and ending as stand alone applications. Author have compiled a nice list of various board versions that include different PIC microcontrollers, different PCB types that can fit in various applications like for plugging to breadboard, or very small SOIC board for tiny projects. Author also provides a list of various firmware versions along with bootloaders. To make things more easier there are few demo applications available for testing or modifying to your own needs.