A record sum of money is expected to be spent by businesses desperate to cash in on the "pass it on" phenomenon this year. But are they the best ads yet, or something more sinister?
Advertisers are reaching new customers through viral marketing
With cheap video manipulation packages available for PCs and Macs, it does not cost a fortune to rustle up a film that can cause quite a stir.
The homemade movie of the guy with the broom handle that looked like a Star Wars lightsaber - you know the one, you must have seen it by now - is one of the most downloaded clips ever. It spread largely through word of mouth.
The term viral refers to how the content - be it a joke, picture, game or video - gets around.
Our e-mail contact lists are always at our fingertips, so it is easy to see how something can quickly get passed on to thousands.
If people want to push the forward button after watching a video file, and recent research suggests one in five UK internet users do, then it is viral.
It is the internet's biggest talent contest.
In the last two years websites dedicated to hosting the sort of content that might go viral, have themselves become internet traffic hubs.
YouTube now attracts more viewers than MTV, and that means a potential goldmine for advertisers.
What happens next? Outtakes from Click are being spread virally
Outrageous and Contagious was billed as Europe's first conference dedicated to viral marketing.
In a world where we skip TV ads, block pop-ups, and bin junk mail, companies are falling over themselves to get their brands sparking reactions, even if it means emulating the usually darker, funnier, edgier content of the web.
Mazda recently commissioned a web ad that was made on a shoestring budget; the car company is said to have preferred the clip to its more expensive TV commercials.
Ironically, it was quickly transferred to the telly in five European countries.
"Now you're seeing a situation where you've got much bigger advertisers putting much larger mainstream brands into this space," said Will Jeffery of the Maverick Viral Agency.
"They're spending a lot more money on production and distribution, but also the creative and conceptual side of actually what it is they're trying to do and how they fit into this space."
Rather than leave it to chance advertisers buy space on viral hubs; the practice is called seeding.
For around $8,000 (£4,300) a week the ad gets placed on the front page, guaranteeing a large initial audience.
Another trick used by the industry is to avoid pushing the brand into our faces - sometimes it is not mentioned at all, which can lead to confusion as to who is the behind what.
"When we speak to clients we say to them 'don't over brand because people don't go to your game because it's a Panasonic game,'" explained Oli Christie of Inbox Digital. "So with Panasonic, for instance, we asked them not to have a huge logo on the front page, to make the product the hero but not to over brand it because they're there to be entertained, to play.
"It is a bit sneaky. You mustn't forget that brands aren't doing this for fun, they're doing this to sell products.
"I think the exciting thing about the internet is that individuals in their bedrooms can take an existing viral, like the Wassup ad or the Mastercard 'priceless' advertising campaign and create their own versions of it.
"It's very dangerous also for clients, because Mastercard has been adapted many, many times, sometimes to quite risky things."
When US car manufacturer Chevrolet invited people to make their own ad about its new model, some saw an opportunity to criticise the company.
Chevrolet declared the campaign a success after five million people visited its site, but playing in the viral space where the gosh factor is king, is a risky business for an industry used to being in control of its image.
"Advertisers who go entering into the viral space have to understand the space has different rules," said Mr Jeffery.
"If you put a campaign out there it will remain out there for years and years. So you have to work very hard to make sure you're saying the right thing about your brand, because you won't be able to stop it once it's out there."
Dan Brooks was responsible for last year's VW suicide bomber car ad, which outraged Volkswagen.
The company had nothing to do with the ad and demanded that Mr Brooks apologise for any offence, and take it off his website.
But millions of people saw it, and you can still find it on the net today. So does the unregulated nature of the internet mean that anything goes?
"It's up to us, as the creatives and the makers of these virals, to police it ourselves," said Dan Brooks of Lee and Dan.
"I think there is almost an unwritten rule that there are things you don't do. There might be people who do these in their bedrooms who might cross the line, but if it's in total bad taste then it isn't a very funny idea and it will die a death, it won't get sent on."
Viral artist Neil Hepburn worries that in the quest for a bigger wow factor things can go too far.
"I've done poor taste gags on the web, and I think I've got near the line, but some people push it beyond that.
"When it gets into racism, or mocking the disabled, then it can immunise people to the reality of what those topics are all about, and I think it's dangerous and I don't like it."
In the UK the Advertising Standards Authority uses the same guidelines for web material as for TV.
It has only ever received one complaint about a viral ad. Whether that is because the British do not know they can complain, or we are aware of how little can be done if we do, is hard to say.
Perhaps we are even more comfortable than the advertisers are with seeing the unpredictable, sometimes violent, and often risque content in our inboxes than we would ever put up with on the box.