The c't journal has a column c't-lab. It provides the complete know how to make a computer controllable multi purpose instrument for measuring and signal generation, a programmable source/sink and other components that are required for research and development of electronic hardware hacks. Communication is possible using a port and software/system of your choice (RS232/USB/Ethernet, use just HyperCard, LabVIEW or even a simple terminal programme).The required PCBs and microcontrollers can be obtained separately (I would strongly recommend the kit-supplier Segor in Berlin), if you do not want to etch the PCBs and to program the controllers to customize them for the given purpose. The author Carsten Meyer sometimes tends to design things a little better than required. In the issue #16/2008 he provides blueprints for some module announced as a frequency counter. In fact it turns out to be a full fledged FPGA module to make anything you can think of using 400000 gates at a rate of 50 MHz or more (Xilinx XC3S400). With help of some knowledgeable fellow of the 68kMLA even strange things might be possible, like this Asteroids emulator with an old oscilloscope tube as a display The c't lab project might be a good start to learn anything you would need to make your personal dream peripheral. Any of the c't-lab modules can be used stand alone, no need to manufacture a multimeter, when you already have one.

Caveats: you will need some basic knowledge of German language, soldering gear for electronics, a few bucks and plenty of time.