Web

For the web, there is Piped (github.com/teampiped/piped) and Invidious (invidious.io)

NOTE: Only Piped proxies your videos, which means invidious allows connections directly to Google to load the video.

Android

On Android, there's NewPipe and LibreTube which are well known by now.

NOTE: LibreTube uses Piped to preserve privacy and (somewhat) anonymity when watching, thus you'll need to select some piped instance with it.

iOS

On iOS, there's the web frontends and Yattee (github.com/yattee/yattee).

NOTE: Yattee is on the AppStore, but without any sources/instances by default. Look for "instances" in the settings and add a Piped API URL, like https://pipedapi.kavin.rocks to watch videos from YouTube with Piped+Yattee.

Desktop

On Desktop, there is all the web clients as well as FreeTube (freetubeapp.io).

NOTE: FreeTube has an option to proxy everything or just the videos through Invidious to preserve privacy. It is recommended to enable this.