Only once I begin using another Laptop/PC I realize the importance of certain softwares I had used to optimize my Laptop. And when it comes to watching Youtube, I believe everyone should keep two softwares with them
- Internet Download Manager
What's VideoCacheView? Well, do you know every time you watch a Video or listen to music[be it on Youtube, Facebook or any other site] the file gets temporarily stored in the Cache of your Browser? VideoCacheView lets you view these files and Copy them to a folder. So pretty much you can watch a Video, if you like it, open VideoCacheView and copy it! As simple as that! Here is a screenshot of the Software. You can download it from here - download.cnet.com/VideoCacheView/
Internet Download Manager
I'm pretty sure most of them would be using this Download Manager - IDM the best Download Manager available. It reasonably fastens download speed, and lets you download Youtube Videos as well. When watching a video, you can see a Download this Video button floating at the bottom, clicking on which will download the Video.
So make sure you have Both!