Visual Studio 2005

Released: October 2005
Office Developer Tools has to installed separately
AddinLoader.dll is not signed
Document Level 2003 - Excel, Word (v2 runtime) (will not run in 2007)
Application Level 2003 - Outlook
Visual Studio Tools for Office Runtime required to run solutions built using VSTO 2005
Outlook add-ins uses IDTExtensibility which effectively called the startup method.
Extensibility DLL not Included
Add-ins for Office require Extensibility.dll in order to run, which is not automatically included in the Setup project for Shared Add-ins.

Update 1 (Released: May 2006)
Also known as Visual Studio Tools for Office 2005 SE
Incremental download for VSTO 2005 also known as Visual Studio 2005 Tools for the 2007 Microsoft Office System.
Focused on targeting Office 2007.
If you install this on top of Visual Studio 2005 professional, the 2003 document level functionality is unavailable.
If you install over the top of Visual Studio Team or Visual Studio Tools for Office 2005, the document level functionality is available.
Also the COM specific registration is also no longer required. There is no ProgID or CLSID registration under HKEY_CLASSES_ROOT
There is also no bootstrapper component VSTOEE.dll
When Office sees the manifest entry in the registry it loads the VSTOEE.dll which then loads the AddinLoader.dll
AddinLoader.dll is signed
The application manifest is identical to the one used for Outlook 2003 in VSTO 2005
Application level Add-in model was changed slightly
Ribbon Extensions 2007 - Excel, Word, PowerPoint, Outlook
Outlook Form Regions 2007 - Outlook