No sooner was the switch for Hulu Plus on Wii U flipped than Amazon Instant Video and YouTube given life as well. Both apps are now available on the main menu -- following a small update for each, natch. If you've already removed the icons, you always re-download them for free from the eShop.
I don't use Amazon Instant Video, so I can't give any impressions on that guy. I did however check out YouTube, but I'm disappointed that there doesn't seem to be any way to browse via touch. Instead, you navigate by looking at the TV screen and pushing buttons, with the GamePad only displaying video descriptions. Until another update arrives, you're better off watching YouTube vids on the Wii U's browser.
With the individual video apps good to go, all we are waiting for now is the activation of TVii. If the speed at which these other apps went live is any indication, we won't have to wait too long.