Version 1.47 of the official YouTube app. YouTube stopped supporting Java ME several years ago. This is the newest version I could find. It's very basic, there doesn't even seem to be a fullscreen mode. It also doesn't support screens smaller than 240x320 or touchscreens. I was actually surprised that it works at all.
The YouTube app relies on Java video playback APIs which not all phones have. If your phone can't record and play back videos or play the videos on the YouTube Mobile site at m.youtube.com this app probably won't work either.
Data Connection Required: Yes
Minimum Java ME profile: MIDP 2.0
Screen Size: 240x320
Touch support: No
Reported Working On: Nokia N95-3, Samsung GT-S8500 Wave
Reported Not Working On: Blackberry Bold 9650, LG Rumor Reflex, Nokia Asha 303, Nokia Asha 305, Kyocera Innuendo,
The app "YouTube" (360 KB) is ready to download or send to your phone as filename: "youtube147".