how to download youtube videos

What has happened to video downloads in YouTube? In the Activity window I used to be able to option double click on the video file and it would download so that i can view offline. Now I notice that video files are broken into 1.7MB pieces and have an expiry time (I think(?)). Is this YouTube's attempt at controlling piracy?

So how can someone download these videos for offline viewing? I don't do this to pirate anything. Its simply that its more convenient to have it on my disk for a short period of time so that I can view it when offline or want to have much quicker access to the video e.g. I have been following some tutorials on playing the guitar. Often I need to start and stop and search through the video for a particular segment. If I have to wait for teh video to load from YouTube it takes some seconds or minutes which can be quite frustrating.

Any help is appreciated. Thanks.