Complimentary registration codes for Opera 8 are being given away until midnight Tuesday. The license is good for the life of that particular browser, said Michelle Valdivia, a marketing communications manager for Opera.
Opera 8, which launched in April, normally sells for $39 per license.
"This is a surprise gift for anyone coming to the party," Valdivia said.
The company's anniversary party site includes games, the history of Opera's browser, downloadable music by Opera employees, and an area to chat with Opera Chief Executive Jon von Tetzchner.
"On this day, we celebrate our users," Tetzchner said in a statement.
The Oslo, Norway-based company began in 1995 as a research project at Norwegian telecom company Telenor. Opera develops and markets browsers for desktops,and home entertainment devices.