Developing MS Word add-in

Accepted answer +6 upvotes

There are lots of options for development tools for Office. The most obvious one is of course Office itself. It has rich support for macros and VBA. You could also use SharePoint to extend document sharing and management functionality. But if your add-in is more complex than can be handled inside of Office, I suggest you use Visual Studio 2008 or the Tools For Office add-on for Visual Studio 2005.

One thing to keep in mind is that Office is mostly a collection of COM objects. So while tools like Visual Studio, with its deep support of the .NET Framework and Office classes make it very simple to develop solutions for Office applications, with some time, energy, and a high tolerance for pain, you could develop an Office add-in with Notepad.

Microsoft has a very nice resource site for Office developers here.