While creating a web part page I discovered that the Query String Filter web part was missing. In fact, none of the filter web parts was available. A search for “filter” revealed that the filter web parts belong to a site-level feature called BizAppsSiteTemplates. However, for some reason, this feature wasn’t activated. Furthermore, it had the Hidden attribute turned on so I couldn’t see it in the site feature list. I tried to manually install the feature using STSADM, but it turned out the feature was alread installed. So I activated it with STSADM:

After that the filter web parts finally showed up in the Web Part Gallery.

Once you locate the DWP file for a web part, you can also upload it directly to the Web Part Gallery. But activating the parent feature is more efficient and elegant.