The first movie heavily promoted virally online ?

Snakes on a plane is #1 box office movie this week (thanks, Romain) and it looks like its success is mostly based on the buzz on the web. Some explanation on the fresh wikipedia page:

"Snakes on a Plane became an Internet phenomenon soon after Josh Friedman's blog entry and mentions on several Internet portals. The title inspired bloggers to create songs, apparel, poster art, pages of fan fiction, parody films, mock movie trailers and even Short film parody competitions. Snakes on a Plane has also inspired the creation of graphics for fictional movies about other animals in odd settings, such as "Bears on a Submarine" and "Sharks on a Roller Coaster" (Tagline: You must be this tall...to DIE!).

Snakes on a Plane was promoted highly on the Pod-cast Filmspotting, formerly known as Cinecast.

The phrase "Snakes on a Plane" has been used on Internet forums to indicate that a given topic is nonsensical. With creative uses of capital letters, bold or italic text, and punctuation the title has been manipulated to reflect surprise, horror, or absurdity, among other things. The meme is often interspersed with images of Samuel L. Jackson reprising his role as Mace Windu and quoting lines recalling his roles from both the Star Wars series and Pulp Fiction. The slang form "SoaP" is used in place of "Shit happens" or "Oh well, what'cha gonna do?"
An independently produced promotional image for Snakes on a Plane.
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An independently produced promotional image for Snakes on a Plane.

A viral video entitled "Snakes on a Plane Early Auditions" features comedian Dave Coyne doing impressions of actors Christopher Walken, Jack Nicholson, Joe Pesci, and Robert De Niro, and muppet Beaker, doing casting auditions for the movie.[16] The film has been featured on Digg, iFilm, YTMND, and YouTube, and was included as enhanced content on the film's soundtrack. Dave's voice acting talent can be heard in the audiobook dramatization of Snakes on a Plane."

update: a good article, the first Internet Joke ever made into a movie