Before the announcement of Portal 2, Valve went back and made a few subtle changes to the original game. They added radio frequencies which most people would probably ignore, and added a few seconds worth of material at the end of the game. It probably took a few days to pull off, but the response to it was overwhelming.
Thousands of people got involved with trying to work out exactly what all of it meant and it ended up spreading the word about Portal 2 before the release of a single word about a sequel, and certainly no trailer or screenshot. Jason Holtman gave a talk to a room of industry execs about just how Valve succeeded in pushing the idea of a sequel successfully without the need for massive campaigns or mammoth advertising budgets, just a little bit of cleverness and a great community.
In the end Holtman said that this very "Valve" way of doing things ended up being "better than buying a billboard advert on the side of the Chrysler building."