Is there an open source USB to JTAG circuit I can include in my PCB design?
There are many examples on the web of using an FTDI FT232R chip in its "synchronous bit-bang mode" to drive a JTAG interface to configure a microcontroller or CPLD/FPGA, including the necessary host software. One advantage of this approach is that you can subsequently switch the chip to its regular UART mode and use it to communicate with the target application.