An open source USB stack for PIC microcontrollers has evolved in the forum over the last year. It supports a number of USB-enabled PICs, and a good developer would test each chip before each release. We’ve been crowd sourcing that testing in the forum, but now prolific forum member Mats is designing a PCB so stack developers can switch between different microcontrollers easily.
In essence it is a pcb with a single USB connector and a single ISCP and then jumpers to connect them to a particular chip on the board. A reset button and bootloader buttons with status leds might come in handy too. The clock frequency is selectable between 48MHz and 1.5MHz in 15 rather common frequencies.