Answer: Today, most companies prefer using open-source automated testing tools that will enable them to cut down the cost and effort that manual testing involves. Selenium is one such open-source automated testing tool for websites. Performance, execution speed, and browser interaction of Selenium are faster than any other automation tool. The test packages, the modes available for unattended execution and the ability to develop the test script in any language supported by Selenium make it the most popular automation testing tool. In addition, Selenium supports different operating systems along with support for different programming languages such as Python, Pearl, Ruby, PHP, .NET, C# and Java, and various browsers, thereby making Selenium the leading automation testing tool.