The Facebook App Center, on which a lot is riding for the company, is now available for all users. The feature was introduced a couple of months back and the site started rolling it out. It's been a quick rollout too, compared to some of Facebook’s other new features. The Timeline for example is still not available to all, despite being almost a year old.
But the App Center is, you'll notice it in the left sidebar next to your News Feed. In the App Center, you'll find all sorts of apps, both on Facebook but also on the web at large and even mobile apps. The only thing they have in common is that they have some sort of Facebook integration, they use the Open Graph, even if it's only for logging in.
For users, it's an easier way of discovering apps you may like. If done right, the App Center could actually improve your experience on both Facebook and your phone and tablet. For Facebook though, the App Center is a chance to get some control over the app world on platforms it can't otherwise, i.e. mobile.