You can build one yourself. Assume the BeBox geek port:
One "GeekPort" (37-pin D-shell)
An experimental-electronic-development oriented port, backed by three fuses on the mainboard.
Digital and analog I/O and DC power connector, 37-pin connector on the ISA bus.
Two independent, bidirectional 8-bit ports
Four A/D pins routing to a 12-bit A/D converter
Four D/A pins connected to an independent 8-bit D/A converter
Two signal ground reference pins
Eleven power and ground pins:
Two at +5 V, one at +12 V, one at -12 V, seven ground pins.
My current favourite are STM32 discovery boards, i.e.:
http://www.st.com/web/catalog/tools/FM116/SC959/SS1532/PF254044
(costs $10)
It even has a User USB port, you can use. Then you have to invest some time implementing the whole
communication chain to your PC. ST generally has a standard peripheral driver library that can help you
with all the stuff you can do with the chip in question.
Apart from the powersupply stuff the bebox offers, you will find everything else and more in the microcontroller.
If you want to build an external geekbox you would want to add a separate power supply. Then you could use the microcontrollers PWM outputs
to regulate some voltages for a power supply output, or use some dc/dc converters for fixed voltages.
From the top of my head I wouldn't know what to do with it though, but I do need a power supply unit, some measurement means, and some stimulation means for circuits.