12. A computer system comprising: one or more processors; memory storing data and one or more programs for execution by the one or more processors; the stored data and one or more programs comprising instructions for: setting a respective reuse flag for a corresponding document in a plurality of documents wherein the respective reuse flag is based on a document importance score associated with the corresponding document, wherein the document importance score associated with a document is based, at least in part, on a query-independent metric of an importance of the document for a search engine; and performing a document crawling operation on the plurality of documents in accordance with the reuse flag for respective documents in the plurality of documents, including reusing a previously downloaded version of a respective document in the plurality of documents instead of downloading a current version of the respective document from a host computer in accordance with a determination that the reuse flag associated with the respective document meets a predefined criterion.
18. The system of claim 12 , wherein the document importance score associated with the document is based, at least in part, on a number of other documents that reference the document.