Open Source Symbolic Math Packages?

3Suns asks: "There are many proprietary symbolic mathematics suites available, including Mathcad, Mathematica, and Maple. Strangely, I can't find a single free software project with similar functionality. These programs are as ubiquitous in universities and engineering companies as they are expensive. Given the deep roots of open-source in higher education, what can explain the lack of free/open-source mathematics software?" We last addressed this question three years ago, but a lot can change in that time. Has it?