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ClipGrab is a free Mac app that lets you save YouTube videos to your computer. ClipGrab is a good alternative to QuickTime because you won't have to sit through the video while it records—you'll just enter the URL and the app does the work. You should be able to click its name at the bottom of your browser. If you don't see it, double-click the installer in your Downloads folder.  It'll be in the Applications folder. It's near the top of the window. If you haven't already done so, bring up the video in your favorite web browser now. To copy the URL, click the address bar to highlight it, and then press ⌘ Command+C to copy. To do this, click back over to ClipGrab, click the typing area, and then press ⌘ Command+V to paste. If you have a different video file preference, choose that instead. It's below the URL you pasted into the blank. ClipGrab will download the YouTube video to your computer's default Downloads folder.
Go to https://clipgrab.org and click Free Download. Open the ClipGrab installer. Drag the ClipGrab icon to the Applications folder to install. Open ClipGrab once the installation is finished. Click the Downloads tab in ClipGrab. Copy the address of the YouTube video you want to download. Paste the copied URL into ClipGrab. Select MPEG4 from the "Format" menu. Click Grab this clip.