One of the relatively hidden but often invaluable features of OS X is its support for services, which is when one application can provide a function or capability to another, and accept some input such as text, files, or images, and perform a separate task with this data.
For instance, if you are in Safari and wish to send a selection of text to a friend in an e-mail, you can click and drag to highlight it, and then use a service to create a new e-mail that contains this text. Likewise, you can select text in Word, Pages, TextEdit, or other applications and similarly generate an e-mail message containing the text, using the same service. In this case, the program offering the service is Mail.
The services in OS X are available primarily in the Services submenu of the application menu (immediately to the right of the Apple menu), but can also be accessed in the contextual menu of programs like the Finder and Safari.
When you install an application in OS X, often services it can provide to other applications are collected and made available in this menu, so the options available on one computer may be different than those on another computer. However, Apple does provide a collection of services in OS X, some of which are enabled by default and others that can be toggled, if desired, or even bound to custom hot keys for quick access.
To see what services your system has available, go to the Keyboard system preferences and select the Services section from the Keyboard Shortcuts tab. This will show you the various services available for handling pictures, messages, selected text, files and folders, options for searching, and more.
To add a service to the Services menu, simply check the box next to it, or uncheck it to disable that service. By doing this, you can customize which services are available to you, and keep the Services menu clean and efficient. For example, if you are not an application developer, then you might not need any of the Development services that OS X provides, so disabling these might help.
In addition to accessing services through the Services submenu, you can define custom hot keys for them, so frequently used ones can be quickly accessed by a simple keystroke or two. Some of Apple's built-in services may already have hot keys assigned to them, but you can reassign these, or add new ones to services that do not have a hot key.
Unfortunately, when adding hot keys sometimes it is easy to double up on a previously used one, so one recommendation I have is to use the Control, Option, and Command keys together for custom hot key assignments. These modifier keys are rarely used together for built-in commands, and are easy to all press at once, which makes them convenient for a quick custom hot key (i.e., you will not have to remember whether your hot key used Control-Option, or Option-Command, etc., though this specification is certainly possible as well).
Finally, you can also create your own services. This is one of the more exciting aspects of OS X, as you can tie together various features of otherwise separate programs. You can create a service that only applies to data and objects in one program, or one that applies more globally to all programs.
The means for creating your own services in OS X is Apple's Automator program, where you can use a collection of predefined actions to assemble a workflow for manipulating data in stereotyped manners. For instance, you can create a service that batch-renames a number of selected files, append a file to a Keynote presentation, or move all selected items in the Finder to your Documents folder to help keep things organized.
To create a service, open Automator and create a new workflow, and select "Service" (designated by the gear icon) as the type of workflow to create. Then choose the type of input (text, rich text, files, folders, or both files and folders, and so on) that the workflow will handle, and optionally specify the application in which this workflow will apply. For some inputs like text, you can further refine the type of text such as phone numbers, URLs, or addresses, if desired.
When the inputs have been created, then you can assemble your workflow from the various actions available, just as you would any other Automator workflow. For advanced users, you can set up Applescript and Shell scripts with the workflows, along with Automator variables to highly customize the actions. Keep in mind that while Automator was intended to be simple, sometimes routines may take a bit of thought, trial and error, and research to get done.
With the workflow completed, save it with a descriptive name, and it should now be available in the Services section of the Keyboard system preferences, where you can enable or disable, or assign your own custom hot key to it.