Mac OS X
Apple advertises the Services menu in connection with other features of its operating system. For example, it's possible to desktop search for a piece of text by selecting it with the mouse and using the service from Spotlight. Other central services are Grab for taking screenshots, and the system spell checker. The concept is similar to a GUI equivalent of a Unix pipe, allowing arbitrary data to be processed and passed between programs.
Services can be implemented as application services, which expose a portion of the functionality of an application to operate on selected data, usually without displaying an interface. In its developer documentation, Apple recommends that applications use services to provide features that are "generally useful", giving as an example a Usenet client providing ROT13 encryption as a service. Standalone services may also be created without a host application. Their simple, one-purpose nature and the fact that they don't require a GUI to be designed makes writing standalone services popular beginner's Mac OS X programming projects
Since many applications install their entries without asking the user, the OS X services menu tends to clog up with dozens of entries quickly. Most users only will ever use a small subset of the possible options, therefore cutting down and customizing the menu makes it both faster and more pleasant to use. Prior to Mac OS X Snow Leopard, third party software is required to do this; in Snow Leopard, the Services menu can be customized from the Keyboard pane of System Preferences.
Read more about this topic: Services Menu