Tubbler is a Mac program that enables you to watch YouTube clips directly from your desktop. The difference between using Tubbler instead of watching Youtube videos with your default browser is that the aforementioned desktop application doesn't use the Adobe Flash Player, hence, it uses less CPU usage while playing the clips.
The utility lets you choose between two methods to import YouTube videos: drag-and-drop the URL from your browser or use the Safari extension. Either way, the app takes a small amount of time to load the selected clips and lets you choose the preferred playback quality (240p, 360p, 480p, etc.).
You can create playlists by dragging YouTube URLs over the app's main window, enable Shuffle mode if necessary, and play clips in a fullscreen or windowed mode.
However, I wasn't happy about the fact that I still needed to use the default browser in order to import YouTube videos. The application should include an integrated search tool in order to ease the import process.
Another aspect I don't like is that the developers offer you a Tubbler extension only for Safari. Browsers like Mozilla Firefox or Google Chrome are not supported.
Briefly, even with two important flaws, I'd still install this app on my Mac. It significantly reduces the amount of CPU resources your system uses for playing YouTube clips. Plus, it's free of charge.