SAN FRANCISCO — Google Inc.'s free e-mail service will shed the final remnants of its invitation-only restrictions today, extending the reach of an increasingly popular product that has emerged as a vital cog in the online search leader's expansion efforts.

Invitations will no longer be required to join the nearly 3-year-old "Gmail" service in the United States, Canada, Mexico and a swath of Asian and South American countries where the Mountain View-based company previously limited the number of users.

With those restrictions now lifted, Gmail will be open to all comers worldwide for the first time since Google unveiled the service on April Fool's Day in 2004.

"It's a pretty momentous time for Gmail," said Keith Coleman, Google's product manager for the service.

Although it will no longer require invitations to sign up, Gmail is retaining its "beta," or test, status, signaling that Google still considers the service to be a work in progress.

Making Gmail more widely available is important to Google because other key products like instant messaging and calendar management are tied into the e-mail service, company co-founder Sergey Brin said an interview. "It has become a real cornerstone for us."

Because Gmail users often remain logged into Google's Web site while they conduct online searches, the service also helps the company's engineers learn more about individual preferences — knowledge that can help deliver more relevant search results and foster more loyalty.

The decision to lift all invitation requirements on Gmail signals Google finally believes it has adequate computing capacity to accommodate the generous amount of free storage provided by the e-mail service after investing heavily in additional data centers. Gmail offers each account at least 2.8 gigabytes of storage — enough to fill about 1.4 million pages.

In 2006 alone, Google's capital expenditures totaled $1.9 billion, with much of the money going toward additional computing capacity. It's an investment that Google could easily afford, having earned $3.1 billion on revenue of $10.6 billion last year.

Now that Google has more computing muscle, Brin said the company will start selling additional storage capacity to e-mail users with extraordinary needs. Google still hasn't figured out the specifics, but Brin indicated the e-mail storage and fees to be introduced later this year would be similar to Google's photo-hosting service that charges $25 annually for 6.25 gigabytes and $500 annually for 250 gigabytes.

"We can't afford to give away everything for free," Brin said.

Google tries to make money off its e-mail service by electronically scanning the content of the communications so it can display advertising links tied to the topics being discussed.

Gmail's advertising methods have raised some privacy concerns and turned off some potential users who don't like the idea of their e-mail discussions being perused or commercialized.

Nevertheless, Gmail has been growing rapidly as Google gradually opened the service in other parts of the world and made it increasingly easy to wrangle an invitation where the restrictions were still in effect.