21. A system comprising: one or more server devices comprising: a search engine to: perform a search of a corpus of documents to return a plurality of documents, where the plurality of documents includes a first document, determine whether a measure of freshness associated with the first document can be determined, identify, when the measure of freshness associated with the first document cannot be determined, a set of second documents that each contains a link to the first document, determine a measure of freshness associated with each document of the set of second documents, assign a freshness score to the first document based on the measure of freshness associated with each document of the set of second documents or the measure of freshness associated with the first document, where the measure of freshness indicates when each document was last modified, and rank the first document among the plurality of documents based, at least in part, on the assigned freshness score.
21. A system comprising: one or more server devices comprising: a search engine to: perform a search of a corpus of documents to return a plurality of documents, where the plurality of documents includes a first document, determine whether a measure of freshness associated with the first document can be determined, identify, when the measure of freshness associated with the first document cannot be determined, a set of second documents that each contains a link to the first document, determine a measure of freshness associated with each document of the set of second documents, assign a freshness score to the first document based on the measure of freshness associated with each document of the set of second documents or the measure of freshness associated with the first document, where the measure of freshness indicates when each document was last modified, and rank the first document among the plurality of documents based, at least in part, on the assigned freshness score. 23. The system of claim 21 , where the search engine is further to: assign a first freshness score to the first document if a majority of documents of the set of second documents correspond to documents that have freshness values that are less than a threshold; and assign a second freshness score to the first document if a majority of documents of the set of second documents correspond to documents that have freshness values that are greater than or equal to the threshold, where the first freshness score is less than the second freshness score.