All you have to do is pick which site you want to get the download from, add the URL for the video, and then click Download. You'll automatically get a list of all the available options and formats, including MP4, FLV, 3GP, and WEPM. You can also save the video without audio or even extract just the soundtrack.
Just clicking on the desired option makes SaveDeo auto-download your video and save it to your default Downloads folder.
As of now this webapp lets you download from more than 15 different video hosting sites. And in the case of YouTube you can even do multiple downloads of playlists, channels, or user videos.