Marc Andreessen developed Mosaic together with Eric Bina in 1992 and 1993. Mosaic was the first browser to really bring the internet to the masses, thanks to its easy installation and usable interface; an interface which has remained more or less unchanged ever since. Mosaic was also the first browser to render images inline. Contrary to popular belief, however, Mosaic wasn't the first graphical browser.
Still, Mosaic became popular, and in the end formed the basis for Netscape, whose parent company was founded by Marc Andreessen. As we all know, Netscape played a major role in popularising the web, a role it had to relinquish to Internet Explorer. Netscape still lives on though through Firefox, the browser which reignited the browser wars thanks to being a better product than Internet Explorer 6.
So, Marc Andreessen knows his browsers. He was a key player in the history of two of the most important browsers, so when he decides to put his weight behind a new browser project, you can be sure people will notice. RockMelt, as the project is called, is currently nothing more than a web page, and Andreessen is very reluctant to say anything about it. What we do know is that several people from one of his former companies - Opsware - are involved, and that at least one of them worked at Netscape too, back in the day.
Just yet another browser project to keep an eye on.