Name: SageMath
Homepage: www.sagemath.org
Why should this be included in the repository?
It is an important piece of software for those learning and teaching Mathematics as it offers a free replacement for expensive software such as Mathematica and Matlab. From its website:

SageMath is built out of nearly 100 open-source packages and features a unified interface. SageMath can be used to study elementary and advanced, pure and applied mathematics. This includes a huge range of mathematics, including basic algebra, calculus, elementary to very advanced number theory, cryptography, numerical computation, commutative algebra, group theory, combinatorics, graph theory, exact linear algebra and much more. It combines various software packages and seamlessly integrates their functionality into a common experience. It is well-suited for education and research.

Is it Open Source (yes/no): Yes
How many users do you anticipate will use this software?
I really can't. I teach privately in a small classroom of about 20 students and we'd all use it enthusiastically.
Link to source tarball/zip file: source available here