Since REST has been adopted for a long time now it seems that applications integrations are the next step. Usually I thought that integrating different applications should be done via an ESB I am sure now that it is not true.
In my opinion any decent application should have different interfaces exposed to the world. One may consider WebServices and some others REST or any other technology or interface. To keep the focus on integration I will not discuss what is better even if I think that one should use the right tool for the right job.
Once an application has a REST interface it is very easy to access that information directly from the browser for example, even if you can use it on the server side too. The information provided might be in different format such as XML or JSON. JSON is very suitable for browsers and XML is better handled by the server side.
At the end the integration is very easy to be done. It is true that security is a weak point in such an integration but that can be solved too.
To enable REST in your application it is good to consider the integration of a server side scripting language and of a templating framework. The templates should allow you to output different formats such as XML or JSON.