As of this week, registered users are able to subscribe to any categories of their choice. This means that an email notification will be sent to you whenever a new post is submitted in that category. You either subscribe to an individual category from the category page, e.g. Books & Magazines or subscribe to more categories from your Subscription Settings Page.
Do yell out if there are any problems.
Subscribe to category seems useful, but why can't I subscribe to a tag as well? For example, for me, the mobile category is a bit too general, but the Android tag is not.