Gmail just keeps getting more powerful!! Announced yesterday was the ability to view PowerPoint files and Tiff's!! using the same viewer as the PDF one. Origininally you could click on View as slideshow to see the PowerPoint (only .ppt is supported, pptx currently is not) files, which would open a new browser window and would look the same as viewing a Google Docs Presentation. The advantages that the new viewer over the old one has is that you can copy and paste text, print the document, and zoom. This new viewer also allows easy editing, all you have to do is simply click Edit Online.
This is a great feature and will save the hastle of having to download attachments, having the right software, and having to switch between web browser and desktop software.
On a minor note, the PDF viewer restrictions for documents have been lifted/increased. Now you can upload PDFs that are longer than 100 pages (size limit of 10MB) and you can have more than 100 PDFs (not sure what the limit is on this one).