This is a free sample add-in (ready to install and use) that adds a number of useful features to Microsoft Excel, including advanced options for Excel styles, merging cells and special paste options.
The add-in is based on the innovative Add-in Express technology and gives you an example of developing Excel add-ins, creating custom toolbars and controls and accessing Excel objects.
The add-in contains a full source code in VB, C# and supports Excel 2003 and Excel 2007. It works in VSTO 2005 SE and VSTO 2008 (VB.NET, C#).