Microsoft and AOL/Netscape's stranglehold on the browser market is being challenged by Norwegian vendor Opera Software, which has finally made its browser freely available.

Although still available ad-free for $39, a new version of the Opera 5.0 browser with a banner ad embedded is now being marketed.

"The ad-feature will not slow Opera down, nor diminish the internet experience," said Hakon Lie, chief technology officer at Opera.

"The banner-sized ad is quite small, and they are downloaded into the browser only once a week, to be cached for later use. This ensures the recognisable Opera-speed in the browsing itself, as well as not too intrusive ads. Also, every necessary precaution has been taken to ensure privacy."

Although critically acclaimed as fast and functional, the web browser has failed to take off globally. However, even at $39 a time, it has still built up an installed base of 1.5 million users.

Now, Opera said it is planning a major push, distributing "millions" of CDs on magazines around the world, as well as targeting independent portals.

Dean Kakridas, manager of strategic alliances at Opera, said: "With their latest releases, Microsoft and AOL/Netscape have focused on strengthening the integration of their browser, portal and ISP services, putting them in direct competition with independent portals, ISPs and contents sites from around the world.

"This offers a pure browser developer like Opera Software great opportunities to serve these companies fully customised, co-branded versions of its browser."