There are plenty of browser plugins that allow you to download videos from websites like YouTube. A few of them even allow you to convert the video to a different format to suit your needs. However most of them are either complex, require further processing, or simply break every time the browser gets updated. Thankfully, there’s a free Mac OS X app that does it quickly and easily.
Evom from TheLittleAppFactory does just that. Evom allows you to convert online video to a number of formats, including iPod, Apple TV and HTML5.
Evom converts Internet videos to a number of convenient formats easily
The easiest way I’ve found is to drag the Favicon of the YouTube page containing the video you wish to convert to the desktop, then to the Evom window. Evom does the rest for you. In my tests, it worked on YouTube and a number of other sites perfectly.