FxSolver – A solver for engineering and scientific equations
many people have tried to build open source alternatives to mathematica for decades (sage, sympy) and it's just too developer intensive a task. there is so much work and not to mention IP in just Simplify[] that I don't know why people bother. no one that seriously uses something like mathematica (e.g research) would use any other CAS. I'm not saying I wouldn't love a free alternative but it's not practical without dozens of people across the mathematical and engineering spectrum. the challenge that comes to mind is that undertaken by the teams that contributed to numpy, scipy, and pandas in order to be a viable alternative to Matlab.