Windows sharepoint services extensions for Visual Studio 2008

http://www.microsoft.com/downloads/details.aspx?FamilyID=7bf65b28-06e2-4e87-9bad-086e32185e68&DisplayLang=en

It is required to develop custom webpart or any other custom sharepoint stuff.

A good link :

http://www.mediablend.co.uk/blog/CategoryView,category,SharePoint2007WebParts.aspx