Firefox only (Windows/Mac/Linux): Firefox extension Nodobe Viewer is an inline document viewer for Firefox intended to replace the bloat of plugins like the dreaded Adobe Reader. Nodobe Viewer supports 38 different file types, including PDF, Microsoft Office documents, and OpenOffice documents. When you're about to download one of these document types but would prefer just a quick inline view, right-click the link and select Open Link in New Tab with Nodobe. The tool works in conjunction with web application Vuzit (pronounced "views-it"), which handles the document conversion and displays the results in your browser.
The downside: You actually have to sign up for a free account with Vuzit to use Nodobe Reader. If you're willing to take that small step, Nodobe is a great little alternative for quickly viewing a document when you either don't have the right application installed or you don't want to fire up bloatware to view it.