Do you mean a circuit simulator?
Last time I used a sim was a few years ago. I used an obscure but very good program called Simetrix (which I notice, from a google, seems to have been commercialised, from its original UK university origins) and a lousy product called Electronic Workshop which was widely marketed in the USA.
Both are basically user friendly front ends for Spice.
Most PCB design / schematic entry programs (Protel, etc, etc) include a similar analog/digital sim function nowadays, necessary to justify their massive prices.
Sorry this is not up to date; the nature of products I design these days makes them unsuitable for simulation.