EDIT (December 17, 2013) Today they announced an Official Youtube Channel for the Roku 3. It’s expected to come to other versions of the device early next year. Finally!
EDIT (April 4, 2013) Turns out the Private channel VideoBuzz works great for Youtube on Roku. Follow that link and sign in to your Roku account to add the channel. Word on the street is the Roku may be getting an official Youtube channel soon.
EDIT (March 30, 2012): Matt in the comments recommends RateRix, which can be found in the Photos and Videos section of the channel store. You can even set up playlists. This seems like a pretty good replacement to the original NowhereMan Youtube channel. Here is a good review I found of the channel. Matt also offers this handy tip: ‘If you search for any music artist with the word VEVO behind it you can access all their HD content.’
EDIT (March 13, 2012): According to user Garivel in the comments you can also add a 1080p Youtube channel here. Be warned, my original Roku recently kicked the bucket so I can’t test this out. Let me know in the comment if it works.
EDIT (February 28, 2012): While new users can no longer sign up to the Nowhere Man’s channel some commenters on this post have found a slightly complicated workaround. If you have the Facebook channel you can post any Youtube link you want to watch to your Facebook wall and then watch it from within the channel under Newsfeed Videos. If you don’t want to spam your friends with random cat videos just set the post to be visable ‘Only to Me.’ Thanks to Paul and Meghan in the comments!
The Roku Digital Video Player is a great little set-top box that brings tons of online video content (including Revision3, TWiT.tv, Netflix, and Amazon). You may have seen my post back in November which outlined a way to use the new Mediafly channel to play Youtube videos. Hours after my post went up the work-around was displayed somewhere along the supply chain and I was without further recourse to pull Youtube clips onto my TV without the aid of a laptop.
Thankfully I did what I normally do, waited for a solution to come by. Hobbiest Roku Channel Developer TheNowhereMan has developed a private channel that allows you to access Youtube videos straight from your Roku. It’s as simple as linking your box to the Roku.com website and entering a Private Channel code. He has full instructions and the code at his site. as well as instructions on adding his other awesome channels like Pod TV and Woot!.
By the way, you can always find experimental channels to put on your Roku over at the Roku Forums.