A site Philipp co-created is featured in a post on the YouTube API blog today. Here's the intro:

<< A common way to use the YouTube API is to make a niche site – YouTube isn't necessarily built to be a destination site for iguana lovers, or antique car racers, or people looking to learn Mandarin. Developers can use the API to create a rich supporting feature set for these communities. It's great to come across sites like this that are done well, so when I saw WatchToLearnChinese.com, I asked one of the creators, Philipp Lenssen, editor of Google Blogoscoped, to tell us more about the site. (I was also excited because it was a good way for me to brush up on my rusty Mandarin in time for a trip to Beijing!) >>

From: http://apiblog.youtube.com/2008/09/spotlight-on-watchtolearnchinesecom.html

Original Google Blogoscoped post:
http://blogoscoped.com/archive/2008-07-07-n72.html