System Requirements: Windows XP and up; .NET Framework 3.5 SP1
This program is targeted primarily for the educational audience, but it can be useful for jobs that require math calculations. It packs in many features, including equation-solving, unit conversion, 2d and 3d graphing, algebra, trigonometry, and even calculus. It can display step-by-step instructions on how to solve a problem, which can be very helpful to students who are learning new math topics. You can also scribble things free-hand as input, and it does a fairly decent job at recognizing everything (although sometimes you may need to redraw some symbols). This program was originally not free, but Microsoft released the newest version as freeware on January 11, 2011. Microsoft also provides Microsoft Mathematics Add-In for Word and OneNote, which adds Microsoft Mathematics features to those programs.
Download Link for Microsoft Mthematics 4.0