Want to grab videos from YouTube? Get Tube for Mac is designed to do exactly that. Get Tube for Mac supports multiple files for simultaneous downloading. The app installs easily and adds a new option to the menu bar. There's no integration with your browser, but a standalone app. An interface to the browser would have been a nice addition.
When you find a video you want to download on YouTube, simply click on the video and start the download process, choosing the format you want to use (both MP3 and MP4 formats are supported, and you can also select just the audio to download if you wish). The download manager lets you cue up downloads from YouTube and trigger them when you want, a nice feature. The download process doesn't seem to be full-speed, but controlled by the app itself.
The problem we ran into with Get Tube for Mac is that it would sometimes display an error message saying that no video was found when we selected the Download option. This seemed to happen fairly randomly, and not always on the same video after refreshing either Safari or Firefox. When videos were located, the downloads went ahead fine. There are other video download apps on the market, so Get Tube for Mac has to excel to be popular. The lack of browser integration and the failures on some videos were enough to have us wondering if there were better options.