Office Web Apps 2010
I am truly excited with the potential and possibilities of Office Web Apps 2010 (comes along with Office 2010 Volume License). Users with just a browser can edit the document and save it right back on their SharePoint Server. Document formatting everything is preserved just as it is. Very useful for Secure and "reduced" environments....
Deploying Office Web Apps 2010 should be right after your SharePoint 2010 Installation and there are certain steps to be followed, which are explained quite well in technet - http://technet.microsoft.com/en-us/library/ff431687.aspx - Just follow them carefully and you will be pretty fine.
After deploying when creating Word Service, PowerPoint Service applications, you will find your Excel Service Application already existing if you have installed SharePoint 2010 Enterprise Edition.
Not just this, if you are planning FAST 2010, FAST gives document previews right in the search results pane if you have Office Web Apps 2010 deployed :)
Cool isn't it!
I just recorded this video to demonstrate this.
Go ahead and enjoy!