21. A system comprising: one or more computer devices to: select a first document; represent a first relationship between the first document and a second document as a first link between the first document and the second document, where the first relationship includes at least one of: the first document and the second document sharing a same hierarchical user-defined label, the first document and the second document being associated with a same sub-directory, or the first document and the second document being associated with a same bookmark folder; represent a second relationship between the first document and a third document as a second link between the first document and the third document, where the second relationship includes at least one of: a hierarchical user-defined label associated with the first document and a hierarchical user-defined label associated with the third document being sibling labels, a sub-directory associated with the first document and a sub-directory associated with the third document being sibling sub-directories, or a bookmark folder associated with the first document and a bookmark folder associated with the third document being sibling bookmark folders; and assign a rank score to the first document based on the first link and based on the second link, where the first link contributes more to the rank score than the second link.
21. A system comprising: one or more computer devices to: select a first document; represent a first relationship between the first document and a second document as a first link between the first document and the second document, where the first relationship includes at least one of: the first document and the second document sharing a same hierarchical user-defined label, the first document and the second document being associated with a same sub-directory, or the first document and the second document being associated with a same bookmark folder; represent a second relationship between the first document and a third document as a second link between the first document and the third document, where the second relationship includes at least one of: a hierarchical user-defined label associated with the first document and a hierarchical user-defined label associated with the third document being sibling labels, a sub-directory associated with the first document and a sub-directory associated with the third document being sibling sub-directories, or a bookmark folder associated with the first document and a bookmark folder associated with the third document being sibling bookmark folders; and assign a rank score to the first document based on the first link and based on the second link, where the first link contributes more to the rank score than the second link. 26. The system of claim 21 , where the one or more computer devices are further to: determine, for the first document, a number of other documents that share the same user-hierarchical label with the first document, are associated with the same sub-directory as the first document, or are associated with the same bookmark folder as the first document; and where the rank score assigned to the first document is further based on the number of other documents.