Youtube videos can be sent by opening or sharing links with Android Airplay.
Mp4 videos can be sent by opening them in the Gallery or sharing them to Android Airplay via the share menu.
It should work on tablets the same way it works on phones, although I don''t own a tablet to test.
Tested on a Nexus 4 running Android 4.3
More features coming in time :)
On some phones you have to keep the screen on or the video will stop playing after a few minutes.