We recently released the Widget Developer Kit to anyone interested in developing a widget. TVs with the Yahoo! Widget Engine started shipping in March of 2009 and the WDK was released in January 2010. Yahoo! created several widgets but we expect to have more available as 3rd party developers build and submit for QA. We recently launched several new widgets for LG TVs (eBay, CBS, MyCast, Mediabox). If you have not downloaded the WDK you can get it at: http://connectedtv.yahoo.com/developer/#frmContact Fill in your info and hit "Get WDK"
You do not have direct access to other widgets in the QA process or testing. It would be up to the developer to distribute their widget to you for testing.