I realise that this is a forum dedicated to Excel, but I hope you don't mind me letting you know about an alternative that has just been released. It uses the same 'familiar' interface as traditional spreadsheet programs like Excel and Open Office, but is based on a very different model.
Resolver One 1.0 is now available and is free for personal/non-commercial use (and the free version is also an evaluation version for commercial use of course).
Resolver One represents spreadsheets as programs, so it is much easier to use code with the data and formulae in the grid. It allows you to use any .NET libraries natively, plus any Python libraries (including placing arbitrary objects in the grid), making it much easier to develop with.
Resolver One isn't aimed at replacing Excel for all uses (Excel is a very sophisticated program!), but it is a platform for the rapid development of business applications - typically the 'high end' of Excel users. It allows you to bring programming good practise to spreadsheets (maintainability, refactoring, testability!) and so on. It is also very easy to experiment with (with an output console for spreadsheet debugging).
I'm one of the developers of Resolver (so I'm hopelessly biased of course!), and I've put online my own website with articles and hints and tips
on using Resolver (mainly from a techie point of view).
Resolver One has some great features, like database worksheets, worksheet level formulae, indexing worksheets and cell ranges by header name rather than by index. We are also working on some interesting stuff - like simultaneous editing of data (shared worksheets) by multiple users.