So, it would seem this has definitely been abandoned now. It hasn't worked in over a year and the developer in incommunicado. I just hope they are okay.
As far as a replacement for this goes, I have tried several. iTubeX is good, but it does have to boot another app, so it's not the most streamlined option.
For Safari users, I would HIGHLY recommend the Youtube5 Safari extension: http://www.macupdate.com/app/mac/35606/youtube5-safari-extension
It basically converts all video content (such as Youtube videos, which run with Flash) into HTML5 content. It does so instantly and automatically, and results in faster loading videos and less CPU usage in playing them. It's fantastic.
What's more, you can easily download the videos by clicking on the [i] in the top right corner of a video, where it shows you a list of sizes and formats the video is available in. Just right click on your preferred size/format and choose "Download Linked File".
It's so elegant. Just a 21k file. A little Safari extension. No mess, no extra apps, not even a button to clutter up your tool bar.
What's more, it's free!