15. A method comprising: receiving, by a computer system running a tagging mashup application, a new email message sent to a user using an email application; comparing said new email message with previous emails sent to said user; generating, by the computer system, a keyword set of related words, including words found in both said new and previous email messages; presenting, by the computer system, said keyword set to said user to generate a tag; and propagating, by the computer system, said tag across multiple applications by associating said tag with identified entities in applications other than the email application such that displaying one of the identified entities in the applications other than the email application results in displaying the tag together with the one of the identified entities, the identified entities corresponding to entities pointed to by content in the new email message.
15. A method comprising: receiving, by a computer system running a tagging mashup application, a new email message sent to a user using an email application; comparing said new email message with previous emails sent to said user; generating, by the computer system, a keyword set of related words, including words found in both said new and previous email messages; presenting, by the computer system, said keyword set to said user to generate a tag; and propagating, by the computer system, said tag across multiple applications by associating said tag with identified entities in applications other than the email application such that displaying one of the identified entities in the applications other than the email application results in displaying the tag together with the one of the identified entities, the identified entities corresponding to entities pointed to by content in the new email message. 16. The method according to claim 15 wherein said keyword set uses social tagging information from another system besides said email application.