There are tons of tools out there for downloading YouTube videos within your browser, but Get Tube is one of the more frustrating ones I've used.
Get Tube works independently of your browser in a separate YouTube-only browsing window and is designed specifically for Macs. It can save into FLV, MP4, AVI, MP3 and HD format where possible - so far so good. However, using Get Tube can be an absolute pain. The theory is, you search for a video in YouTube and hit the 'Download' button in the bottom-right corner of the interface and that's it. Using the Preferences in the top-left corner of your screen, you can also specify where you want to save files, the format and whether to save HD where possible, and check for automatic updates.
In reality though, when you click 'Download', half the time you get an error message saying no video is found. This error message seems to be random and some videos work while others don't. Secondly, you have no control over the application once it's started downloading. There's no way to pause downloads or even click on another YouTube video to watch while you are downloading. I couldn't even quit the program and had to go into Activity Monitor to force-quit it while downloading. However, Get Tube remains an awfully popular download on the Net, albeit with many people reporting mixed results. The message seems to be, this app works successfully for some users and not for others.
Get Tube either isn't functioning very well or hasn't been tested properly. To save yourself a lot of headaches, I'd recommend using something like Video DownloadHelper which not only works properly, but it works directly in Firefox too.