The line-in of a sound card could be used as an additional source type, and the line-out as a probe. A special "real time simulation" would then simulate the circuit taking data from the former and writing the results to the latter. This could be very useful for digital control.
JACK could provide a good front-end solution for this.
In case the computer isn't fast enough to do the calculations, a zero-order hold would be needed (i.e. repeat the last calculated data until a new one is calculated).