For quite some time, Clear Office has enabled software developers to host spreadsheets inside their WPF application.
Starting with version 1.0.2010.0710, Clear Office supports hosting WPF elements inside a worksheet. Clear Office supports hosting any descendant of FrameworkElement including any control, shape or panel. Here is a sample screen shot in Clear Office:

In this screen shoot, the button filled with an orange ellipse is a WPF button.
You can save spreadsheet documents with embedded WPF elements to a .xlsx file and open them with Excel. Here is a screen shot of the same file in Microsoft Excel

Rountripping is also supported: you can save files from Excel and reopen them in Clear Office.
How it works
Before this release,, worksheet anchors could contain only the following:
- Shape, Picture, GroupShape and Connector from Office.Drawing
- GraphicFrame from Office.Chart
This release introduces a new option for anchored content: XamlElement. XamlElement in turns contains a Content of type FrameworkElement.
Here is the code to add someElement inside a anchor of a worksheet
OneCellAnchor anchor = new OneCellAnchor(worksheet[3, 5]){Width=100, Height=100};
anchor.Content = new XamlElement{e.Content = someElement};
worksheet.Anchors.Add(anchor);
XamlElement uses the standard WPF XAML serialization (XamlReader/XamlWriter) to serialize to file. As a result, any property / attached property and behavior will be serialized.
For example, have a look at the "EmbeddedObjectsSample" Sample of the Spreadsheet Framework. This sample demonstrates how to insert the button in the screen shot above and how to assign a click behavior similar to an Excel macro.
(This is a release candidate, details may change before a formal release)