In real-world circuits, performance is affected by component value tolerances (1%, 5%, 10%); designers want to use cheaper components if they wish to mass produce their products. In radio applications, especially UHF and microwave, parasitics cannot be ignored and must be built into a generic model of the circuit being simulated. In both these cases it is usual to perform Monte Carlo simulations which are difficult or impossible to calculate by hand.
SPICE was originally developed at the Electronics Research Laboratory of the University of California, Berkeley in 1975 by Donald Pederson. Versions 1 and 2 were coded in Fortran (2G.6 in 1983 was the last) but version 3 and later are coded in C.
The original SPICE program was released under a restrictive license, which makes it difficult to improve upon the original software. A new circuit simulator, based on SPICE, called ng-SPICE (for next-generation) is licensed under the GPL. Development on the main branch of ng-SPICE arrested around 2001, but there is an active branch called tclspice. If you want a free SPICE that works on windows, consider LTSPICE.