How do I send a YouTube video from my desktop to my Android tablet? Apps such as YouTube and Netflix use the X protcol to conect to smart TVs so you can send (a link to) the video you're watching to the TV, and it starts playing on the TV right away. I'm looking for something like the reverse of this: if I've started watching a video on the YouTube website on my Ubuntu PC, I'd like to have that start playing on my tablet (e.g. so I can take it with me to the kitchen as I cook). Ideally, it should start playing on the tablet at the same point I was at on the website.

The simplest workaround I can think of would be to use the "Watch later" playlist to save the video, and then open it on the tablet. That's quite long-winded, and I'm hoping there's something easier.
The Chrome to Phone extension is what you'll be wanting.

To get Chrome to Phone, install the Chrome to Phone extension in your Google Chrome browser and the Android app on your phone, and you’re ready to go. 

Chrome to Phone extension adds a button to your Google Chrome browser that instantly sends the current YouTube video, web page, map, or selected phone number or text to your Android device running Froyo (or Android 2.2) or up.

For Firefox users, there's a compatible Firefox addon FoxToPhone, which sends URLs etc. from Firefox on the desktop. It still needs the same app installed on the phone.