Template tag to list themes in the sidebar?

I’ve noticed on a lot of the sites that I was looking for themes on they have a list of the themes in the sidebar and any use can choose a different theme and switch to it. I know how the links are done (the page with “?wptheme=Theme+Name” at the end) but how can I generate the list of themes? I tried looking in the documentation wiki and there’s no mention of theme template tags that I could find that would get this list. Thanks for any help