One of the most powerful features of NeXT’s operating system, which is often ignored in OS X, was that of services—capabilities that one application would export to all others. Services live in their own menu in GNUstep. (In OS X, they’re hidden in the application menu.) They fulfill a range of different uses; a mail client might provide a service to send the selected text in an email message, or a TeX service might replace the selected text with a typeset equation.
One example is the GNUstep terminal, which provides "terminal services" to all other applications. This allows any shell command to be turned into a service. For example, the sort command can be used to sort any selected text, or something like calc, dc, or bc can be used to evaluate selected expressions.