I found on the net that there should be some kind of applet for playing video from youtube directly in Amarok. However those post were more than year old and I didn't find anything newer.

Does this still exist somewhere? Is it possible to use it in the newest Amarok version?

There's a "video clip" widget

you use the wrench icon at the bottom of the middle panel to add/remove/move the available widgets

Oh, I thought it was just for opening youtube in external browser. I always clicked on the link bellow. However when I right-click and use Append and play, nothing happens, new item appears in playlist but I can't play it.

Clicking on the link below the video causes it to open in Konq for me. Clicking on the video itself causes Amarok to skip to the next track in the playlist. I'm using a git version from yesterday.

Very similiar to my case. Clicking on the video itself causes Amarok to jump to the first track in the playlist. However I would like to play the video inside Amarok. I saw some screenshots so it should be possible, I just don't know how.

This was fixed in the 2.3.1beta. In the 2.3.1beta announcement they say "Fixed Youtube videoclip broken engine due to their recent change", it works for me

Awesome. But I'd rather wait for stable release as this feature isn't that important to me I was only interested wheter this still existed when I posted this thread.