LTSpice is a free SPICE simulation tool from Linear Technologies.
For those in the electronics world, SPICE simulation can be a great way to answer the “will my circuit work as advertised” question without breadboarding. It’s a great sanity check- if it don’t work in SPICE, it won’t* work on a breadboard.
LTSpice is Linear Technologies’ own incarnation of SPICE. It’s a nice little product, and comes with models for many of Linear’s products (power supplies, op-amps, etc) along with many supporting components from other manufacturers (diodes, transistors, capacitors, inductors, etc) and generic parts (555s, LEDs, etc). Obviously, they provide it expecting people to download it, sim up a solution involving Linear parts and then sell eleventy-billion a year, thus providing a nice income stream for Linear (and it works- LTSpice makes it much more likely that I’ll reach for a Linear part for power supply design than another manufacturer’s parts).