Windows/Linux/Mac: SpeedCrunch is an extremely fast and lightweight calculator that supports extended expressions. Instead of pecking away entering your equations line by line, you can enter the whole thing in at once. SpeedCrunch calculates in the background while you're entering equations, which helps in locating obvious errors as you work. It also supports syntax highlighting, so you can see immediately if you left a parentheses out. It has support for unlimited variables with multi-character names so you can create values for everything from X to SpeedOfAnAfricanSwallow; additionally, a handy ans variable is a quick way to reference the result of the last calculation. SpeedCrunch has a built-in library of mathematical constants, variables, and equations for common calculations of volume and area of 3D and 2D objects respectively. Fortunately with the number of variables available, there's a snappy search function that is keyboard accessible. SpeedCrunch is freeware with cross-platform support for Windows (available as a portable application), Linux, and Mac. Thanks DevourerKwi!