8. The non-transitory computer storage medium of claim 6 , wherein cached copies of the content items included in the corpus of content items previously delivered to the user device are stored with the index.
6. A non-transitory computer storage medium encoded with a computer program, the program comprising instructions that when executed by data processing apparatus cause the data processing apparatus to perform operations comprising: receiving, from a user device, a search query for a content item, the search query including one or more search terms; identifying a corpus of content items previously delivered to the user device, wherein the content items included in the corpus of content items are advertisements corresponding to content sponsors; creating an index of the corpus of content items previously delivered to the user device, wherein the index of the corpus of content items previously delivered to the user device is populated of a cookie stored at the user device; using the received search terms to search the index of the corpus of content items; determining that at least one content item in the index corresponds to the received search terms; and delivering, to the user device, a content item search result, the content item search result including the at least one content item corresponding to the received search terms.