It's one of the most widely used electronic circuit simulator. It's basically a graphical interface to wire up differential equations, together with physical models of components (mostly semiconductors.)
There are other SPICE implementations, but Linear Technologies provides this program for free, and lots of manufacturers provide LTspice models for their components (of varying quality.)
Next on wishlist is someone doing the same for encrypted fpga verilog blobs.
In other words, obfuscation.
There are other SPICE implementations, but Linear Technologies provides this program for free, and lots of manufacturers provide LTspice models for their components (of varying quality.)