Peek

Peek is a browser extension that lets you preview dozens of types of documents before you download them. Just hover over a file's link, and Peek will display an interactive preview of the file instantly. Peek also allows you to expand a preview to a new browser window for easier viewing.

Peek supports previews for the following file formats:

* PDF and RTF Files
* Word Documents (.doc, .docx)
* Excel Documents (.xls, .xlsx)
* PowerPoint Presentations (.ppt, .pptx)
* WebM, MP4, GIFV, and OGG/OGV Video
* MP3 and WAV Audio

In addition to direct file links, Peek also supports previewing links to public Google Docs files, Imgur GIFV links, and Gfycat links.

Peek is open source under the MIT license. It's code is available here:
https://github.com/corbindavenport/peek