RealDownloader is perhaps the quickest and simplest online video-downloader for Windows that we've seen. Unlike many of its competitors, when RealDownloader says one-click, it seriously means just one click.
When you install RealDownloader, it integrates with your browsers (Firefox, Chrome, and IE are supported), so you don't have to boot it up every time you want to use it. It just detects when you're on a supported site, and pops up whenever you roll over the top-right corner of a video.