Opera proprietary web browser that has a version that is distributed free of charge. Opera development is focused on providing a highly standards compliant browser full of advanced features.
Many features that were pioneered by Opera such as tabbed browsing have made their way into open source browsers like Firefox and Konqueror.
Opera, unlike Microsoft or the Mozilla Foundation, seeks to make a business from developing a web browser. Towards this end, Opera has in the past charged for their browser, or embedded advertising in the browser, however Google has more recently come to a deal with Opera to pay for the search engine defaults, this enables Opera to offer a browser free of charge.