YouTube does not officially allow downloading videos directly from their platform for offline viewing, except through their paid YouTube Premium service78. However, there are some third-party tools and methods available for downloading YouTube videos, though it's important to note that using these may violate YouTube's terms of service.

Some popular online YouTube video downloaders include:

1. SaveFrom.net: Allows downloading videos in various resolutions and formats1.

2. Y2Mate: Converts YouTube videos to MP3 or MP4 files for free2.

3. Media Mister: Offers a free YouTube video downloader with different quality options3.

4. YT1D: Provides a free online downloader compatible with multiple devices and platforms4.

5. Zeemo AI: Offers video downloading along with features like automated captioning and subtitle editing5.

6. SocialPlug: Provides a free YouTube video downloader with various format and resolution options6.

When using these tools, keep in mind:

• Always respect copyright laws and only download videos you have permission to use.
• Be cautious of potential security risks associated with third-party downloaders.
• Some tools may have limitations on video length or quality.
• The legality of downloading YouTube videos varies by country and intended use.

For a safer and officially supported method, YouTube offers offline viewing through its mobile apps for YouTube Premium subscribers78. This allows you to download videos within the app for offline playback, though the videos remain within the YouTube ecosystem and are not permanent downloads to your device.