Proteus can be very expensive if you want to go legitimate though. A single VSM license is upwards of $200. In most cases it's cheaper for a hobbyist to just buy the components and test out the circuit than to run the simulation.

That being said... it's super useful tool. I used a bootleg version of Proteus during undergrad. Coming from a third-world country where electronics labs are often severely under-equipped... it went a long way towards helping me understand the material I was covering.