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For example, I see there are questions with the emacs tag on Stack Overflow, but there are also such questions on Super User, Unix/Linux, and other sites in the network.
Is there a way to follow a tag across all of the Stack Exchange sites, so that one can see the global activity of a topic without visiting all the sites separately?
Yes, there is. Global tags are supported on the Stack Exchange site. Click on the "tagged questions" link in the top navigation bar to create your own tag filter.
This post on the Stack Overflow blog talks about this feature, but lots of people still don't know that it exists. The jury is still out on how we might better draw attention to and make people aware of this particular feature from the individual sites. Please be sure to upvote that question if you agree, and share any suggestions that you may have!
Yes. The feature is called Tag Filters, or the StackExchange™ GlobalTag MegaBlender™ if you prefer. I'll provide a summary, because I can't find a canonical new-feature or FAQ post, but you can refer to the Improved Tag Sets blog post, the tag-filters tag, and the globaltag-megablender (yes, I'm serious) tag for more information. The Tag Sets on Stack Exchange describes the justification for the feature as implemented in the previous version, where they were called Tag Sets.
Filters allow you to aggregate collections of questions from multiple sites. They're a very powerful feature. Each filter can contain many rules for the collections.
You can collect an entire site to a filter. This is useful to keep an eye on several smaller sites.
It's rather impractical for larger sites like Stack Overflow. You can even collect all questions on all sites as in the EVERYTHING filter, but that throws about 700 questions per day in your direction. Good luck.
You can collect all questions across the network with a tag. You can even use * for wildcards!. For example, here's a collection of all the embedded, embedded-linux etc. questions.
As with collecting an entire site, you can choose a subset of sites to add to this filter.
The numbers in parentheses indicate the number of questions on that site that would be added to the filter. Note that you can't add a site on which the tag doesn't exist yet.
It's important to note that "All sites" actually means "All main sites". To get the meta sites, you need to check the 
checkbox.
Once you have your set of rules:
Click "Preview" to get a look at the most recent questions that match your filter:
Don't forget to save changes when you're done! This looks like a good set to me, so I'll give it a name of "Embedded Stuff" and save it for later. It now appears in the sidebar:
You can accelerate this process by starting with someone else's filter. Just choose "Save to my filters":
There are three ways to collect the questions from the filter.
The first is to use the filters page directly:
Note the tabs in the upper right that allow you to sort by hot, activity, newest, and no answers (which is different from the questions->unanswered tab, which is different from the Unanswered button).
There are also statistics in the sidebar for the average questions per day, total views, users following, and email subscribers, which leads us to the next thing to do with a filter:
You can subscribe to your own filters by checking the box in the edit view.
You can't edit others' filters, so use the sidebar or envelope to subscribe to filters you find, or use the "Save to my filters" button to copy it to your account.
At the lower right corner of every feed page is an RSS icon.
You can use this with an RSS reader like Thunderbird or Google Reader, or insert it into a Stack Exchange chat room.
