Wikipedia
Lightworks is a professional non-linear editing system (NLE) for editing and mastering digital video in various formats, including 2K and 4K resolutions, and television in PAL, NTSC, and high-definition formats. Lightworks was an early developer of computer-based non-linear editing systems, and has been in development since 1989.
The program is currently available for Windows, Linux and Mac OS. The development of an open source version, as well as ports to Linux and Mac OS X were announced in May 2010. No source code has yet been released, but the project roadmap has indicated that source code release will happen eventually.
The source code release is currently overdue: The Lightworks website FAQ states "Once the Linux and Mac versions are released the source code of Lightworks will become available. We cannot give a date when this will happen, but announcements and updates will be regularly posted on the Lightworks Forum."