In Visual Studio 2005 we have a component called Visual Studio Tools for Office. Using VSTO, you can build add ins for Excel and Word, but now you can also build add-ins for Microsoft Outlook.
If you've tried developing Outlook add ins before, you'll know what sort of hoops you need to go through, not just to build it but to deploy it too. With VS2005, it all gets easier.
To get a better understanding of this, there is a video at Channel 9 which gives you a great overview of building add-ins for Outlook.