If I feel that it is necessary (or schedule-advantageous) to have a partial prototype before the first PCB version, I tend to create it by combining evaluation boards for the technologies involved, or stand-ins for them. For example, a micro-controller board might get a hand-wired SPI interface connecting it to an FPGA board, or perhaps a wiring harness is created to bit-bang some module from a PC parallel port (often using what is on its way to becoming the device firmware, only with a different low-level I/O wrapper).

Once you have a product line started, PCB's from existing versions of the product (or even a product from a different line that uses some of the same technology) can often be used as a prototyping platform. That can mean anything from simply trying out new firmware, to using them with eval boards, or making a little adapter to replace one chip, or even just using the PCB with all but one component removed, as a breakout board for that chip.

While if pursued to far it can easily become a distraction, there can at times be a use for an in-house capability to make simple PC boards on the order of carriers for particular ICs or small circuits to add additional functionality (or correct oversights). I've worked places that did this with milling/laser resources that were required for special purpose RF boards, which we sometimes leveraged for small logic modules. For personal projects, a small toner-transfer single sided SMT board can be produced in one hour and assembled in a second - it's not the quality of a quick turn PCB house, but quick-turn does not mean same-day circuit operation. I'll sometimes send the file to the board house and then make my own version to keep the project moving.