1. A method performed by one or more server devices, comprising: receiving, by one or more processors of the one or more server devices and independent of any search query, a selection from a first user of one or more documents, where the one or more documents are used to form a plurality of custom content groups; indexing, by one or more processors of the one or more server devices, the one or more documents to form a custom search index for each of the plurality of custom content groups, where each custom search index is different from a web search index and any other custom search index associated with the plurality of content groups; receiving, by one or more processors of the one or more server devices and from a second, different user, a selection of one or more of the plurality of custom content groups; receiving, by one or more processors of the one or more server devices and from a client device associated with the second user, a search query; performing, by one or more processors of the one or more server devices and based on the search query, a search of the web search index to identify web search results; performing, by one or more processors of the one or more server devices and based on the search query, a search of one or more of the custom search indexes associated with the selected one or more custom content groups to identify custom search results; generating, by one or more processors of the one or more server devices, a search result document that includes the web search results, the custom search results, and a plurality of advertisements presented within at least a first area and a second area of the search result document, where the first area is distinct from the second area within the search result document, where the web search results and one or more of the custom search results are included within the first area, and where the advertisements and another one or more of the custom search results are included within the second area; and providing, by one or more processors of the one or more server devices, the search result document to the client device.