Users can't open office docs

This one plagues SharePoint installations.

Users can't open document in libraries from a link, they get multiple prompts, nothing happens

See problems here:
http://support.microsoft.com/kb/932118
http://support.microsoft.com/kb/871155
http://blogs.msdn.com/b/sharepoint/archive/2007/10/19/known-issue-office-2007-on-windows-vista-prompts-for-user-credentials-when-opening-documents-in-a-sharepoint-2007-site.aspx

See solutions here
http://www.uv.mx/personal/gvera/2010/09/28/sharepoint-login-prompt-when-accessing-files-in-a-document-library/
http://stackoverflow.com/questions/375390/office-documents-prompt-for-login-in-anonymous-sharepoint-site