Play it once, Sam is a free Chrome add-on developed by Sören Köhler. It is designed to enhance the functionality of YouTube's watch later list by allowing users to play individual videos without automatically advancing to the next title.

The add-on addresses the problem of YouTube's default behavior of automatically playing the next video after starting a single video from the watch later list. This can be inconvenient for users who use the watch later list to collect interesting videos that may not be related to each other.

With Play it once, Sam, when a user starts a single video from the watch later list, it redirects the playback to the single video player instead of the playlist player. This means that the user can watch the selected video without worrying about the next video in the list automatically playing afterwards.

The add-on also provides the option to configure the single play behavior for all playlists or for watch lists only.

Please note that the add-on is no longer being maintained, but it is still available for use. Alternatively, the developer recommends using Tampermonkey and a script provided in the GitHub repository for a similar functionality.