Source code (C#) at http://www.itu.dk/people/sestoft/corecalc/. Doesn't look actively maintained ("A dump of the Funcalc 0.14.0.0 source code as of 27 September 2014, as a Visual Studio 2012 solution. Binaries and examples are in SDFCalc/CoreCalc/bin/Release/. You will need NET 4.5 or later to run Funcalc.exe. When I find the time, I will further clean up the source code, add more method documentation, and move the source to Github to facilitate sharing and branching.")

Also of interest:

* EuSpRIG (European Spreadsheet Risks Interest Group): More about the application than the tech

http://www.eusprig.org/

* Ray Panko's Spreadsheet Research Website: Outdated, but some interesting stuff

http://panko.shidler.hawaii.edu/SSR/

EDIT: Edited a little

Thank you. I was looking for something just like this on spreadsheet implementations, inner workings, models etc.. If you have any more resources like this please post them as well.

The author has written a book (MIT Press), which seems to be a newer (maybe longer?) version of this.

The first couples of chapters looks like a nice resource to introduce the fundamentals of spreadsheets.