Using Smtube for Youtube videos needs updated Smplayer-0.8.3. Youtube has under Christmas changed the video url management once again.
I compiled latest Smplayer-0.8.3 and also Smtube-1.5.
You need also working Qt libs for using them.
I have uploaded Smplayer-0.8.3 and Smtube-1.5 to the pet_packages-upup repo. There is also qt-4.8.2 sfs.
Umplayer would have youtube viewer inbuild but unfortunately it is unmainted atm. 01micko patched the Umplayer couple of months ago with source patch copied from smplayer source. It might be possible to patch Umplayer once again.