Even the old PCs could've done that. The ancient Covox adapter that hid the R-2R DAC ladder, the .mod music that played through it in glorious 8bit mono... the times!
A R-2R DAC, combined with a 74HC595 shift-register, is a nice way to make analog outputs. If all you have are three wires (clock, data, latch), you can still daisychain a fairly large number of analog gauges and digital outputs for LEDs there. (Edit: With SMD resistors you can get away with just one value, and either use the 2R as two in series or the R as two in parallel. That way you buy only one kind but in higher amount, and they are amazingly cheap in bulk.)
Too bad that today's hardware does not have exposed GPIO pins by default. The ol' parallel port was pretty versatile.
With modern PCs, however, you still have a chance. Attach a microcontroller in I2C slave mode to the SMBus or any I2C bus (VGA port, HDMI port (hidden there under the alias DDC), DIMM memory sockets, they all are I2C exposed to the OS)).
A third way is using USB. Either the V-USB library on the microcontroller and software-emulate (which then is a little problematic on the host side as you have to write a service software via libusb, which you may not want to bother with), or use a USB-serial chip (my favorite is the FT232RL). Or if you want barebones functionality and just the shift registers, you can get away with FT245RL and bitbang the three wires.