Concerning  web and desktop apps I also find web services a lot compelling. Talking from a dot net point of view, there is a technology called WCF (windows communication foundation), which is replacing web services in .net. WCF is an ideal way of developing distributed applications(web or desktop apps). You can develop a wcf service that you host and is accessible to virtually all types of applications. In other words you can develop a wcf service that you can host on a web server, a custom host application or a windows service that you can  access  from a browser application  or a desktop app independent of the platform you are using(of course platform independence  depends on the specific endpoint bindings you use). You can change the implementation of your service and need not to worry about the clients as long as you interfaces remain the same.