1- Go to Youtube or Google video or any other online video site, then play the video you want to download, then copy the link in the address bar
2- Paste it in the bar in the site: http://keepvid.com
3- Click on Download link (at the bottom of the page). That's it!
Note that: the file you will download will be in the flv extension so you need a program which can display that kind of videos, I recommend a program called VLC (download it from this link) to watch the downloaded videos by it.