Output 0..5 voltage controlled by PC
Here is one example of a USB to 12 bit DAC for around $80.


  Typical applications include variable air damper control, blower and
  motor speed control, light level control, audio sound level control,
  motion control, water/fluid/air valve position control,
  variable-frequency drive (VFD) motor control, 4-20ma loop control,
  solenoid position control and hundreds of other applications. Connects
  to USB - Powered from USB port 4 channels of 10 bit Digital to Analog
  (1,024 increments in output level) with DAC-4U10 4 channels of 12 bit
  Digital to Analog (4,096 increments in output level) with DAC-4U12
  Selectable reference voltage - allows for 0 to 3.3v, 0 to 5v or user
  supplied voltage output ranges Available with high precision voltage
  reference pre-installed. Allows for (0 to 1.2v), (0 to 2.048v - .5mV
  per increment), (0 to 2.5v), (0 to 3v) or (0 to 4.096v - 1mV per
  increment) output ranges