Links to some really useful recently discovered sites:

OpenSolver for Excel

OpenSolver is an Excel VBA add-in that extends Excel’s built-in Solver with more powerful solvers. It is developed and maintained by Andrew Mason and students at the Engineering Science department, University of Auckland, NZ. OpenSolver provides the following features:

• OpenSolver offers a range of solvers for use in Excel, including the excellent, Open Source, COIN-OR CBC optimization engine which can quickly solve large Linear and Integer problems.
• Compatible with your existing Solver models, so there is no need to change your spreadsheets
• No artificial limits on the size of problem you can solve
• OpenSolver is free, open source software.

Excel Formula Parsing:

• JavaScript version
• Python version
• Other versions

RubberDuck for VBA:

Your IDE is about to enter the 21st century. Rubberduck is a free, open-source COM add-in under active development that integrates with the VBE to enable the features every programmer wants to have in their IDE.
​From unit testing to source control, from code inspections to refactorings, programming in VBA will never be the same.

FormulaDesk:

Add superpowers to Excel
Amazing tools to improve your spreadsheets

And finally, not a new link, but this looks like a really useful article:

Building an add-in for Excel:

Excel is a very powerful application with thousands of options and functions to help you build your models and reports and analyses. Oftentimes your daily job could do with some additional functionality. Something Excel does not offer “out of the box”.

With VBA, augmenting Excel with new functionality is possible. This article demonstrates how I created a small utility. It outlines all steps I used to create the tool. The principles I used in these steps may help you when set out to build your own add-in.