The RSS Viewer web part was not displaying as an option when I tried to add it to a page. Even if I chose the “Advanced” option there was no mention of it.

In order to use the RSS Viewer on a publishing site, you need to go to your top level site, select “Site Actions”–>”Site Settings”–>”Modify All Site Settings”.

Under “Site Collection Administration”, click “Site Collection Features” and enable “Office SharePoint Server Enterprise Site Collection features”.

Thanks to the post here: http://www.sharepointu.com/forums/p/2995/8085.aspx#8085

UPDATE: This fix did not work on the next server I tried it on. There was one more step I had to do.

1. In SharePoint Central Administration, go to Operations.

2. Under “Upgrade and Migration” select “Enable Features On Existing Sites”.

3. Select the checkbox (there’s only one… smashing UI design) and click OK. I had 3 applications with very few sites and it took about 5 minutes.