Minitube is a YouTube desktop application. With it you can watch YouTube videos in a new way: you type a keyword, Minitube gives you an endless video stream. Minitube does not require the Flash Player.
Minitube is not about cloning the original YouTube web interface, it aims to create a new TV-like experience.
For Linux this application is free and open source: "This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version."
For Windows and MacOS there's a free version and a commercial version. The latter includes email support and free updates to all future versions.