According to Bruno Terkaly, you have three main choices:

1. Traditional Visual Basic for Applications (VBA)
   Dates back to all the way to the early nineties.
2. Visual Studio Tools for Office.
   Really hit its stride and office 2007. It probably represents the best option moving forward today. It tries to be all things to all people.
3. The COM Add-In is probably the most powerful, yet the most difficult option of the three.

I’ve borrowed the above words from Bruno.  You can read all about those options in his blog post.