YouTube Downloader+ (not to be confused with YouTube Downloader, from a different developer) is a free, Java-based utility that lets you easily download and save videos from YouTube.
Since YouTube Downloader+ uses Java, you can actually also use it in Linux and Windows--which should provide fair warning that you won't get a slick (or even typical) Mac interface or experience here. That said, YouTube Downloader+ is fast and easy to use, with a straightforward interface window. You just paste in the URLs of the videos you want to download, then hit Return, and they will be saved as .flv files in whatever directory you designate. There's not much flexibility beyond this core function, but you can also run the app from the command line, suspend downloads, and choose from several different languages.
There are other free apps that can perform this same function, including at least one browser plug-in (and some janky methods without using an app at all), but YouTube Downloader+ gets the job done with minimal fuss.