This project has been discontinued, and is no longer being maintained.
Syphon Virtual Screen was an open source app to add a fake extra-display to your mac and pipe its video content to use in your video workflow. This could be used i.e. to hijack video output of applications that can go full-screen but are not Syphon-enabled.
The project has been discontinued because of some recent changes in OSX 10.10 and greater:
Source code is still available for reference on the development branch.
If you are working on OSX before Yosemite (10.10) you can find the installer on the release page.