How to download online videos from YouTube, Youku, Tudou and other video sites

YouTube, Youku, Tudou and other video sharing websites have a large number of movies, animation, video tutorials and other related entertainment and learning resources, sometimes we hope to be able to download them to the local save, the following four ways to download online videos, some browser extensions or plug-ins, some independent third-party software, some of which can be saved. Some of the video sharing website official tools, some download address generation website, everyone can get what they need.