I haven't heard about that site but I'll try to find out what the plan is.
During the beta period we had a site where you could ask questions and posts examples of Extensions - beta.qlik.com You can still find the old discussions and examples at the site but you can't post new questions.
I think we will create a specific forum for Extensions here on QlikCommunity but I'm not sure when it will be ready. In the mean time, post your questions in this forum.
To get going there is an Extension examples document that's included in the QV10 install package together with 4 or 5 Extensions where you could get some ideas about Extensions. If you have any specific questions about how to start, feel free to ask!
I work on a product called GeoQlik which uses the QlikView extensions along with a special server to display server-side rendered maps in QlikView documents. You can view a demo at http://qonnections.geoqlik.com.
My opinion is that the extension API is currently not mature enough to allow robust and fast custom visualizations of data in QlikView without using tricks or workaround. Also, given that they are executed on the client-side in a browser the performance quickly drops when processing a lot of data. Last and not least, the lack of a good documentation is frustrating, there is often no way to know how something works without resorting to trial&error work...
However I think it is a very promising feature and hope it will be improved a lot in future releases.