The Showcase is the part of the site in which we display user submitted skins and allow others to download the skins. The other part of the site displays the default skins in Subtext.
It appears that Tim has been porting some of the nicer designs in the Open Designs website, a website devoted to open source web design.
If you are a Subtext user, try out some of these skins. They may find their way into future releases of Subtext.