John McAfee, the flamboyant anti-virus software industry pioneer, made a surprise appearance at a computer hackers conference on Friday evening, where he unveiled a new website to give people a place online to vent their anger. The site, called BrownList (http://www.brownlist.com/), carries the motto Its payback time. This taps into anger in a positive way, he said in a brief interview before taking the stage at Def Con, the worlds largest conference of computer hackers. Instead of just lashing out, give us your positive solutions, he told an audience of hundreds of hackers taking part in a three-day conference.