This is the first time I noticed this: an email in my Gmail with Excel attachments offers me the option to view the document in Google Spreadsheets.
Of course I tried it and indeed, the spreadsheet shows up in a webbased form. It doesn’t work for all spreadsheets though (I got an error message on the second document), but it is a nice extra feature to Gmail and Spreadsheets.
Now they could add this for Powerpoint (via Picasa slideshow?), Visio and all OpenOffice formats, and they just pretty much made the Word/Excel/Visio viewer applications obsolete. Even more importantly: it works on all platforms. Nice one!