1. A method performed by data processing apparatus, the method comprising: identifying, by one or more data processing apparatus, a set of previous interactive sessions for a particular task, each interactive session for the particular task being an online session during which the particular task was performed; identifying, for each interactive session in the set of previous interactive sessions and based on content of the interactive session, a set of implements used during the interactive session; classifying, by one or more data processing apparatus, at least one implement in the identified set of implements as a required implement for the particular task, the classification of the implement being performed based, at least in part, on a portion of the previous interactive sessions in which the implement was identified; indexing classified implements according to the particular task; receiving a search query that references the particular task; and providing, by one or more data processing apparatus, data identifying the indexed implements in response to receipt of the search query.