The tag control shows up on all documentation that has opted in to the Community Content feature (currently 1.5 million pages across 7 languages). There is a page-level tagging control just below the topic and one added to each Community Content block. It looks like this:
You can use the Recent Edits RSS to find topics for tagging. And Molly Bostic showed me another cool feature: you can subscribe to an RSS feed for any tag or combination of tags. For example, the following feed shows topics and community content tagged with C# and code: http://msdn2.microsoft.com/en-us/library/Tags-Cloud(d=rss).aspx?tag=c%23+code.
MSDN Community Content with tags.