SEOUL -- The winning candidate in last week's South Korean presidential election had little need for mass rallies or traditional campaign tactics. When Roh Moo-hyun's organizers wanted supporters to vote on election day, they simply pressed a few computer keys. Text messages flashed to the cellphones of almost 800,000 people, urging them to go to the polls. During his campaign, millions of voters absorbed Mr. Roh's message from Internet sites that featured video clips of the candidate and audio broadcasts by disc jockeys and rock stars. Half a million visitors logged on to his main Web site every day to...