There are many streaming video sites on the Internet that let you upload/play videos, but most of them do not offer the download option.
All Video Downloader is a good free tool that lets you download videos from almost all video sites including most popular streaming sites on the Internet like Youtube, Dailymotion, Vimeo, Google Video, Yahoo Video, etc.. The site supports over 350 video sharing sites. In addition to downloading the videos, you can choose which format you want to save them.
The application is also compatible with various formats of mobile platforms as iPad, iPhone, iPod, PSP, etc. So you can easily add links and downloads in format you like. There is also support for downloading by batch, you can download multiple videos simultaneously.