After almost three years of waiting, there's finally a YouTube app for the 3DS, available on the eShop. Even though I haven't yet checked it out myself, it sounds a bit crap. I mean, really.
Info
Features: - Video playback in 144p/240p - Possibility to log in with your account & subscribe to channels
What you can't do: - Watch 3D videos in 3D - Post PM's or comments (however you can answer) - Upload Videos - Edit Video data (descriptions etc.) - Enjoy all the good ads you would have on almost every other version of YouTube - Watch Playlist Videos automatically