It's amazing just how much software comes with Mac OS X 10.6 Snow Leopard. Chapter 8 details the several dozen applications, utilities, and widgets. But there's another kind of software in Mac OS X beyond these: helper applications called services.
A service can be specific to an application or general purpose (meaning it's available for several applications). The services available at any moment depend on which application (including the Finder) is running. Apple includes a bunch of services in Mac OS X, and individual applications can add their own (Opera and Skype both do, for example), some of which other applications can use as well (depending on how they are designed).
To find out what services are available, choose Services from your application's menu. For example, choose Finder
The services are displayed contextually based — not only based on the application that is running but also based on what is selected. For example, iWork Pages shows the Look Up in Dictionary, Make New Sticky Note, Send Selection, and Search in Google menu options only when text is selected. And both Numbers and Keynote show Look Up in Dictionary, Make New Sticky Note, Capture Full Screen, Capture ...