Selenium
Browser automationThe long-established open-source standard for cross-browser automation and testing.
Selenium has automated browsers for well over a decade. Through the WebDriver standard it drives every major browser from many languages, and Selenium Grid distributes runs across machines.
Newer tools are nicer to write, but Selenium's reach — language support, browser coverage, and an enormous base of existing knowledge — keeps it relevant.
Where it's ideally used
A fit where the widest browser and language coverage matters, or a team has deep existing Selenium investment.
Where it doesn't fit
More setup and more flakiness than modern tools — rarely the pick for a fresh automation project.