In the world of videogames, Blizzard Entertainment (a subsidiary of Activision) is a -- to put it lightly -- a Big Deal. The rake from their overwhelmingly popular World of Warcraft is enough to put the GDP of some small countries to shame. Even before the launch of their now-behemoth MMORPG, Blizzard made a solid name for itself by releasing well-loved, lore-heavy games in series like Warcraft, Diablo, and the soon to be sequel-ed Starcraft.
The following that Blizzard Entertainment commands is perhaps surprising not just in size, but also in loyalty. Fans of Blizzard's games are dutiful in returning each time a new game in a series is released. Part of this is simply because of the superior gameplay -- by reducing the amount of games that they release, Blizzard has astounding quality control over a relatively small (for their monetary clout) library of games. Part of it, however, also has to do with the fact that Blizzard Entertainment is keenly aware of its target demographic (computer gamers who love science fiction and fantasy) and knows how to rile them into a frenzy for each new launch.
Late June of last year, a "splash page" depicting an iced over area suddenly went up on the majority of Blizzard's game sites, including the popular World of Warcraft home page. This splash page offered no explanation, only the Blizzard logo beneath it.
Day One:

The buzz this simple change created was instantaneous. Forums dedicated to gaming erupted in discussion about what the new splash was for. Wrath of the Lich King, the second expansion for World of Warcraft, was set to release in November of that year and had already been announced. Many people believed that the splash was dedicated to the upcoming game, as WotLK features one of -- if not the -- most notorious villains of the Warcraft universe, and has a strong focus on the freezing tundra of Northrend, where much of the expansion's new content is located.
A few, however, were skeptical. The artwork for much of WotLK had already been available, and the cloak of secrecy surrounding the cryptic splash page suggested a different announcement. The next day, another slightly altered, splash page emerged to take the first one's place.
Day Two:

It became clear that Blizzard was certainly building the tension among its fan base, and the certainty that this was just another ad for the upcoming World of Warcraft expansion began to waver. Blogs began reporting on the teaser "splash" pages, further circulating the mystery and putting the rumor mill to work.
With something akin to the fervor shown by LOST fans, people were already analyzing every little bit of the image. For the next week, Blizzard had a captive audience of people who were already fans of their games, as well as much of the gaming industry.
Day Three:

Day Four:

Day Five:

By Day 5, through a large amount of collective work and some very careful scanning for details, many individuals following the splash page evolution had come to the conclusion that Blizzard was gearing up to announce the third installation in their popular series Diablo. Though difficult to see in these images, people went to lengths to analyze even the runes carved into the ice shards and compare them to both Diablo runes and those found in WotLK.
Day Six:

In just six days an Blizzard Entertainment was able to generate a tremendous amount of buzz in the online gaming community for minimal costs: they ran the splashes over the home pages of their own games and content, where it was picked up by excited bloggers across the world. By this point the general consensus was that an announcement of Diablo III was imminent, but fans were eagerly awaiting the final day in the progression so that they could be sure that they had solved the mystery -- and that a new game in a well-loved series was really in the works.
Day Seven: Diablo III Announced

"And the heavens shall tremble."
Aside from the brilliance of getting a lot of bang for so very little buck, Blizzard actively engaged their fanbase in encouraging them to put together the pieces of the splash-page-puzzle.
The real magic, and sly cunning, goes even further beyond the terrific job of implementing a sort of "viral" ad strategy for the prelaunch of Diablo III. Not only did Blizzard advertise and build buzz for Diablo III itself, but the speculation that the splash pages might be for Wrath of the Lich King also increased buzz for that upcoming game.
This week-long set of teaser pages did double duty in increasing word of mouth (or word of blog) for two videogame launches. And THAT takes finesse.