VBA is the big beast - but here is a question from me. What will it take to implement VBA on top of Lua (or any other lightweight,embedded runtimes/VM). Hell, what will it take to implement VBA on top of the V8 engine [1] ? If one can achieve that, then you can pretty much build a spreadsheet as a browser plugin for 80% of the cases. For the rest 20% pathological cases - fine, reboot to windows.

Now, thats's an idea for a Kickstarter. This is the kind of thing that goes on to transform businesses in third world countries - Excel is what prevents the move to Linux more than ANYTHING else.

[1] http://ramblings.mcpher.com/