In early March, Microsoft introduced Office Web Apps on Windows Live SkyDrive in more than 190 countries that includes viewing, editing, and sharing Office document (Excel sheet, Word document, PowerPoint slide) attachments in Hotmail.

Office Web Apps is being used by more than 30 million people worldwide. Recently Microsoft has rolled out an update (Spring update) to Office Web Apps on Windows Live SkyDrive based on continuous feedback and suggestions.
Some key ingredients from Spring update:
Excel users can inert, delete or rename sheets in a workbook within Excel Web App and also use AutoSum and formula assistance to crunch data more efficiently.

You can now change the design theme of your PowerPoint presentation within the PowerPoint Web App.

Courtesy: Microsoft