For downloading videos from YouTube or other sites, if you're using Safari and have
ClickToFlash installed, right-click on the video and select "Download Video".
You can also use
ClipGrab or
KeepVid or similar apps or sites such as
file2hd.com (for downloading any file types from a site).
To download via Safari 5:
While viewing the video just click "Window > Activity" then identify the video in the list of elements (it will likely show something like "258K of 1.8MB" and will be the largest file on the list.
https://forums.macrumors.com/attachment.php?attachmentid=300852&d=1314650333
Hold the option key down and double-click the file to download.
There have been mixed reports about the availability of this functionality in Safari 6, using the Develop menu.
The quality of the downloaded file depends entirely on the quality of the video at the source.