16. A data processing apparatus comprising: a server computer configured to host a web server; a data storage device coupled to the server computer and storing a plurality of different website resources; in the server computer, a website resource selection unit that is configured to: receive, from a requesting computing device, a request for a resource of a requested website; determine a plurality of context values associated with the request, including a particular context value identifying information associated with a referral website that is different from the requested website; select a plurality of rules, wherein each of the plurality of rules specifies an expected context value, a website resource, and a rank, wherein the plurality of context values contains the expected context value; select a particular website resource from a plurality of website resources, wherein the particular website resource is the website resource of a rule having a highest rank of the plurality of rules; cause the particular website resource to be delivered to the requesting computing device.
16. A data processing apparatus comprising: a server computer configured to host a web server; a data storage device coupled to the server computer and storing a plurality of different website resources; in the server computer, a website resource selection unit that is configured to: receive, from a requesting computing device, a request for a resource of a requested website; determine a plurality of context values associated with the request, including a particular context value identifying information associated with a referral website that is different from the requested website; select a plurality of rules, wherein each of the plurality of rules specifies an expected context value, a website resource, and a rank, wherein the plurality of context values contains the expected context value; select a particular website resource from a plurality of website resources, wherein the particular website resource is the website resource of a rule having a highest rank of the plurality of rules; cause the particular website resource to be delivered to the requesting computing device. 25. The apparatus of claim 16 , wherein the website resource selection unit is further configured to: receive a second request for the same website resource; determine the plurality of context values associated with the second request, wherein the plurality of context values associated with the second request are different than the plurality of context values associated with the first request; select a second website resource from the plurality of website resource based on the plurality of context values associated with the second request, wherein the second website resource is different than the particular website resource.