Here is the list of websites with information about PowerPivot for Excel, PowerPivot for SharePoint and PowerBI

• PowerPivot for Excel - Official Microsoft website for PowerPivot technology
• SQLBI.com - PowerPivot for Excel, DAX and SSAS Tabular in depth articles
• PowerPivotPro.com - Website created by Rob Collie Business Intelligence professional and one of the founding engineers behind PowerPivot, Microsoft’s new self-service BI offering
• Microsoft Books Online - PowerPivot integration with SharePoint documentation
• PowerPivot and DAX forum - PowerPivot and DAX forum from PowerPivot-info.com
• Microsoft PowerPivot for Excel forum - SQL Server PowerPivot for Excel forum
• Microsoft PowerPivot for SharePoint forum - SQL Server  PowerPivot for SharePoint forum