I don't have time to hunt and check for this sort of stuff on my own, but luckily there are plenty of other people who do and post what they find.
The Warner Brothers' marketing juggernaut is turning out a pretty large viral campaign for next summer's The Dark Knight. Since this past July's San Diego Comic-Con they have been launching mini-sites, staging events, and over all trying to build up interest in the movie by creating all sorts of non-traditional promotional pieces.
For instance, recently they have launched a number of new websites. A site for the fictional Gotham Times newspaper, the Gotham National Bank, the Gotham Police department, and even a grassroots organization trying to make Gotham safer.
The Dark Knight Unofficial Movie blog does an excellent job of detailing these and all of the other sites, including breaking down the secrets that are hidden there. For instance, it was on the Unofficial Movie blog that I learned about the new link from the Why So Serious site (a supposedly Joker run website) that points to a personality profiler.
Thanks to the quality of the first film, it's already a given that I will be paying good money to see the sequel this summer. So these sites aren't really needed to convince me to see The Dark Knight. None the less, I love that WB is building out all of these online games and sites. It makes the waiting for the new film a little more fun.