XDA

XDA is the official app from XDA that lets you convenienty access its forums, see all the threads at a glance, leave comments, and basically do everything you would normally do on the site. To enter the forums, you don't need to be registered, but you do if you want to comment.

From the XDA app, you can also easily download XDA Labs, a sort of app store where you can download tons of interesting apps. To download apps, you do need to be registered, though.

XDA is an essential app for anyone who frequently visits this popular forum.