Opera is a web browser developed by Opera Software. While it is proprietary rather than open-source software, it is released as Freeware. Opera is known for usability and standards compliance. It has been designed with security in mind, and versions are available for a wide range of mobile devices. The desktop version runs on Windows, Linux and OS X, and there are mobile versions for Android, iOS and Blackberry.
To run userscripts on Opera, you normally need a manager extension such as Tampermonkey. The choice of userscript manager is affected by the version of Opera you are using. Older versions used Opera's own Presto rendering engine, but recent versions (from v15) use the Blink rendering engine, also used by Chrome. Tampermonkey is only available for Blink-based systems, but the Violentmonkey extension is also available for Presto.