It is now possible to view documents (Word, Excel, PowerPoint, OneNote) that one received through a Office 365 account (so viewing them in Outlook Web App) directly in a browser.
For example, I received an email with an attached PowerPoint presentation:
If I now want to quickly have a look at the presentation without opening it in PowerPoint, I can simply click on Open in Browser next to the attachment to view it in the corresponding Office Web App:
A new window pops up and shows the file in OWA, here PowerPoint:
The big benefit of this is that not the whole file needs to be downloaded in order to see it, having a quick look at the file is much easier and faster.