1. At a computer system, a method for registering a data processing agent to manipulate data of a data processing pipeline, the method comprising: an act of receiving a self-contained assembly that contains instructions for implementing a data processing agent and that contains configuration data indicating how the data processing agent is to be configured, the configuration data identifying a specified event the data processing agent is to hook into; an act of storing the assembly at a specified location; an act of detecting an occurrence of an event; an act of determining that the detected event is the specified event; an act of configuring the computer system to implement the data processing agent in accordance with the contained configuration data in response to the determination; and an act of loading the data processing agent such that the instructions can be executed to manipulate data related to the detected event in accordance with the contained configuration data.