[dublin]

I don't know of anything like this for Windows, Mac, or Linux, but there should be. For years, I used a truly excellent Palm app called MathPad. One great thing about it was that it had a magic integrated solver: you just entered the relevant equations and whatever variables and constants you had, and if you put in enough, it would solve for whatever hadn't been defined with the press of a "solve" button. This is way better than a spreadsheet that can't support ad hoc calculation of any term.

It could handle at least moderately complex systems of equations: I entered all the formulae for the algorithm of Oudin (used to calculate the date of Easter), and it traversed all the equations to reach a correct result. I'd still love to have something like that today, without paying $200! Sadly, I don't think it was open source.