Windows only: Small utility KallOut integrates into popular desktop programs such as Outlook, Adobe Acrobat, Word, Excel, PowerPoint, Internet Explorer and Firefox and lets you search the web for text found there. Simply highlight text inside the application (like PowerPoint) and right click to choose your search engine. Here's a video demonstration of the integration:
Kallout embeds searches for Google, Yahoo, Windows Live Search, Wikipedia, Dictionary.com, IMDB, Amazon, Craiglist, eBay, Flickr, Photobucket, YouTube, Metacafe, Google Maps, Virtual Earth, and dozens more into a simple right click menu that is accessible in programs outside your web browser. KallOut is a free download for Windows only.