WordPress.org had their official theme viewer at http://themes.wordpress.net and it was largely stagnant for more than an year.
Now they have launched a brand new theme viewer !
It does not have many themes as of now, but eventually I believe people would fill it in with their themes.
If you want to upload your theme into the theme viewer, your theme must have a GPL License.
Unfortunately none of my themes are released under GPL, so you would not see my themes there.
[Read this post if you have a little bit more time.]
The only place to see and download my themes is at my Free Themes page and Premium WordPress Themes site.