Browser-maker Opera is shaking up the mobile advertising market just a little, after it introduced Sponsored Web Pass, a new version of its pay-as-you-go Web Pass service that effectively gives phone owners free mobile Internet in exchange for watching sponsored messages.

The Norwegian company, which counts 350 million users worldwide, explains that advertisers work with operators to select from a range of free offers for would-be viewers, such as one day of free mobile Internet access, or an hour of Twitter. The offers can only be redeemed inside Opera’s browser, so this doesn’t mean unfettered access to apps and other services.

Once the advertiser has set the reward in partnership with an operator, users simply show up with their eyeballs and, after the ad finished, enjoy their freebie.

sponsored web pass low res Operas Sponsored Web Pass lets brands give you free mobile Internet for watching their ad

When the free package has run its course, users are offered the chance to pay up for more permanent Internet access, or, if they like freebies, they can seek out another advertising promotion instead.

Opera compares the system to “the way an ad from a sponsor airs before the start of a TV program,” and it’s easy to see how it would be popular among pay-as-you-go users, and particularly those in emerging markets where Opera is strongest. The service is available for brands and operators now, though there’s no word on where (and when) it will roll out to users.