Cypress is a fully supported free add-on to Visual Studio to allow you to build Office 2007 applications with Visual Studio 2005. VSTO “Orcas” will provide additional support including Excel workbook and Word document support and visual designers for the Ribbon and Custom Task Panes.

Details of the announcement

Cypress CTP download (VSTO v3)

And if you’re confused as to the difference between Cypress and VSTO downloads…