Google Subscribed Links are a way to build your own OneBox results, by defining direct answers for a list of queries. For example, you could check the status of a flight or find a book if you know the ISBN by entering a simple query.
Now the subscribed links can have a richer interface by including Google gadgets. You can't add inline gadgets or built-in gadgets like the preview of Gmail's inbox, but the good news is that your gadget can access the query and display something relevant.
A good example of gadget that's now included in a subscribed link is Google Translate. After subscribing to the gadget, you can translate some text directly from Google's search box. A query like [translate "I love you" to German] will trigger a direct answer.
This could be used to show search results from other search engines, the latest posts from a blog, solve complicated equations, show relevant photos from your Flickr albums and more.