StumbleUpon is a discovery engine (a form of web search engine) that finds and recommends web content to its users. Its features allow users to discover and rate Web pages, photos and videos that are personalized to their tastes and interests using peer-sourcing and social-networking principles. Toolbar versions exist for Firefox, Google Chrome, Opera, Internet Explorer and Safari. StumbleUpon also works with some independent Mozilla-based browsers. Native mobile StumbleUpon apps exist for Windows, iOS, Android, and the Amazon Appstore.