Microsoft has really and truly created some buzz for PowerPivot. I have been asked to show the demo of Powerpivot in recent time even when I am doing relational database training. Attached is the few details where everyone can download PowerPivot and use the same.

Microsoft SQL Server 2008 R2 – PowerPivot for Microsoft Excel 2010 – RTM
Microsoft® PowerPivot for Microsoft® Excel 2010 provides ground-breaking technology, such as fast manipulation of large data sets (often millions of rows), streamlined integration of data, and the ability to effortlessly share your analysis through Microsoft® SharePoint 2010.

Microsoft PowerPivot for Excel 2010 Samples
Microsoft® PowerPivot for Microsoft® Excel 2010 provides ground-breaking technology, such as fast manipulation of large data sets (often millions of rows), streamlined integration of data, and the ability to effortlessly share your analysis through Microsoft® SharePoint 2010. Download examples of the types of reports you can create.

Microsoft PowerPivot for Excel 2010 Data Analysis Expressions Sample version 1.0
Microsoft® PowerPivot for Microsoft® Excel 2010 provides ground-breaking technology, such as fast manipulation of large data sets (often millions of rows), streamlined integration of data, and the ability to effortlessly share your analysis through Microsoft® SharePoint 2010. Download this PowerPivot workbook to learn more about DAX calculations.

Note: The brief description below the download link is taken from respective download page.

Reference: Pinal Dave (http://blog.sqlauthority.com)