1. A method comprising: creating, by a computer system including one or more processors executing a cross-service tagging mashup application, connections to multiple applications, each application having multiple entities; determining, by the computer system, relationships between said entities within a single application and between entities across multiple applications; associating, by the computer system, a tag with a selected one of said entities in a first one of said multiple applications; identifying, by the computer system, based on said determined relationships, entities in other applications besides said first application that are related to said selected entity, the first one of said applications including at least one of an email application, a blogging application and a website-bookmarking application, and said other applications include at least a different one of the email application, the blogging application and the website-bookmarking application; and propagating, by the computer system, said tag across the multiple applications by associating said tag with said identified entities in said other applications, the propagating said tag including generating the tag in the other applications such that displaying one of the identified entities in the multiple applications results in displaying the tag together with the one of the identified entities.