Although Opera can be downloaded for free, the unregistered version displays advertisements on each page. Filling out the simple form on the Opera site provides both onscreen and in-e-mail registration codes for the Windows, Linux, Mac, Solaris, and FreeBSD editions of the Web browser, and removes the ads.
The free offer expires at 9 a.m. EDT Wednesday, August 31.
Other activities at the company's Party site include online chats with Opera workers and free downloads of tunes written and played by employees.
"On this day we celebrate our users," said Jon von Tetzchner, Opera's chief executive, in a statement. "From the very beginning, we've consistently worked closely with our community to make sure we deliver the best Internet experience. In short, we would never have been where we are today without all the people who have volunteered their time and energy to Opera."
Opera lags far behind other alternatives to Microsoft's popular Internet Explorer browser. According to a recent survey by NetApplications, Opera's share of the market is a slim 0.49 percent, putting it behind IE (87.2 percent), Firefox (8.07), Safari (2.13), Netscape (1.5), and Mozilla (0.52).