When you read an article, you often find concepts or names that aren't properly explained, but it's inconvenient to interrupt your reading and use a search engine to find more information. KallOut is a Windows application that addresses this problem: whether you read the document in a browser, a word processor or in a PDF reader, you can select some text and obtain some useful information in a mini-browser.
KallOut lists many search engines and reference sites from which you can choose, but it also suggests good options. For example, if you select "DMCA takedown", KallOut suggests an article from Wikipedia, for "Flickr" KallOut points to Flickr's homepage, while "NY" is linked to Google Maps and geotagged photos from Flickr.
The software displays search results, maps, videos, Wikipedia articles, definitions, translations - all without opening a new page. For now, KallOut supports Internet Explorer, Firefox, Microsoft Office 2003/2007, Adobe Reader, WordPad and Notepad.
Internet Explorer 8 introduced accelerators, a similar feature that already has a public API and an impressive gallery, but KallOut is not limited to Internet Explorer, it's easier to use and it requires less clicks.