I am looking for free alternatives to download youtube videos in Safari.
There are a few extensions that will add a nice download button to the Youtube video page, and give you the option to download in various resolutions.
You can also try ClickToPlugin, which does a lot more than just offer a download link, but it does that, and it's one of my favourite extensions. Makes watching web video much nicer.
Although not a safari plugin, a good alternative is the youtube-dl python script which works everywhere.
It can be downloaded on this github page.
If you want to use this directly from Safari then you could create a Service like this with automator (the path to the python script should be modified accordingly)
With this service you can select a URL in Safari (for example in the address bar) and access it from the right click menu or the Services menu.