I use Microsoft Office Outlook 2007 at home* and love this feature. The first time I stumbled upon it I wondered why I hadn’t found it before, only to find it was new in 2007!
* Sadly it’s still Office 2003 at work.
Previewing Attachments in Outlook - Microsoft Office Outlook Team Blog
Next time someone sends you an attachment, instead of double-clicking, try single-clicking on it. Outlook 2007 has a great new feature that previews an attachment…