I would say that open-source is not of great interest in optimization software. The idea is to write as little code as possible, and to trust that everything performance-critical has been optimized. Even speed is not the main issue for me. I want something that is flexible. I want to write little code because the less I write the less bugs there are. Correctness trumps everything else.
If I am using SF to allocate investments, I want to make sure an optimal solution is found, even if it takes a little longer. Computer time is cheap. Buy a bigger computer. Developer time is more precious. There are only 24 hours in a day.
You want to change the source code? With all due respect, but I would speculate that 99,9999% of HN users are not qualified to write numerical optimization code. Looking at it is of little use unless you have a PhD in Applied Math and years and years of experience.
Of limited use to HN readers? To those writing web-apps, perhaps. Those doing Machine Learning will probably be ecstatic to find this.
Solver Foundation is not a port of Excel Solver - it is a ground-up implementation in managed code (C#). As TriinT said, the focus is on addressing optimization problems that commonly occur in finance, engineering, supply chain, etc. It's fair to say that there are things that Excel Solver offers that Solver Foundation currently does not (e.g. nonlinear programming), and things Solver Foundation offers that Excel Solver does not. In particular, there is a roster of solvers that are all accessible from a consistent, .Net friendly API.
It's not open source, but there is a solver plug-in model which allows you to use other solvers in place of the ones supplied by our team. In particular, open-source solvers such as lp_solve are supported.
Nathan
http://hanselminutes.com/default.aspx?showID=209
Regarding similar software, there's MOSEK and a bunch of others whose logos show up on Solver Foundation's website. If you like to code in Python, there's CVXOPT and CVXMOD. If you're into MATLAB, there's CVX and Yalmip.