Windows Marketplace is a bit like the Red Hat Exchange: it's a place to learn about, download, and try applications that run on Windows. It's a Microsoft site that aims to increase use of Windows.
It's not, however, limited to Microsoft and its proprietary friends, as this picture shows:
You can find MySQL on the site, but good luck trying to track down OpenOffice, Alfresco, SugarCRM, or most other open-source projects. Still, the Marketplace is limited to applications that the vendors themselves put on the site, so it's not Microsoft's fault they're missing.
Here I'm allowed everything all of the time, sings Thom. Including open source. On Windows. Who would have thought?