Andrew Breitbart, the outspoken conservative writer, activist and website operator, has died unexpectedly in Los Angeles, where he lived with a wife and four young children. He was 43. Breitbart, who may have been best known as the conservative who brought down ACORN, was pronounced dead at the UCLA Medical Center shortly after midnight on Thursday, the Los Angeles County coroner's office told Yahoo News. The cause of death was not immediately known, a spokesman at the coroner's office said. One of his websites, BigGovernment.com, announced his death early Thursday. "Andrew passed away unexpectedly from natural causes shortly after midnight this morning in Los Angeles," the note on his website read. "Andrew lived boldly, so that we more timid souls would dare to live freely and fully, and fight for the fragile liberty he showed us how to love." It's unclear what those natural causes were. A representative for the website did not immediately return a request for more information. According to the Associated Press, Breitbart collapsed while walking near his home in Brentwood. Paramedics could not revive him. Conservatives reacted swiftly to the news. "I'm crestfallen," Rick Santorum told reporters traveling with his campaign. "What a powerful force. What a huge loss, in my opinion, for our country and certainly for the conservative movement. My prayers go out to his family. I'm really sorry to hear it." More... "RIP 'O Mighty Warrior!" Texas Gov. Rick Perry wrote on Twitter. Perhaps Breitbart's biggest and most recent claim to fame was his outing of New York Rep. Anthony Weiner last June. Breitbart's BigGovernment.com website broke the original story of Weiner's lewd Twitter photo, which the Congressman initially denied was his. But, Breitbart finally forced his hand with the threat of more photos. At Weiner's infamous second press conference where he confessed his improprieties, Breitbart stole the show, preempting Weiner by grabbing the microphone and taking questions from reporters. "I'm here for some vindication," Breitbart barked. "The media says, 'Breitbart lies, Breitbart lies, Breitbart lies.' Give me one provable lie." (Jon Stewart later compared Breitbart's scene-stealing appearance to Kanye West grabbing the spotlight from Taylor Swift at the Grammys.) In 2010, though, Breitbart's credibility had been burned after his website posted video excerpts of a 40-minute NAACP speech by U.S. Dept. of Agriculture employee Shirley Sherrod that appeared to show her making racist comments. Breitbart drew heat when the speech was published in full, showing that selectively edited video had taken the remarks out of context--and Sherrod had been fired for it. (The White House later apologized for dismissing Sherrod, a longtime USDA official, and Sherrod sued Breitbart for defamation, a suit that was ongoing when he died.) Breitbart's early foray into journalism began when he helped launch the Huffington Post before moving to the Drudge Report--where he served as Matt Drudge's deputy--and eventually launching his own cache of right-wing websites, including Breitbart.com, Breitbart.TV, Big Government, Big Hollywood, Big Journalism and Big Peace. And in the hothouse world of online political media, they broke some big news. Breitbart--who once told Slate his "entire business model is to go on offense"--was credited with breaking a series of undercover videos showing alleged malfeasance at the offices of community-organizing group ACORN. "All I can think of at the moment is what Andrew meant to me as a friend, starting from when we worked together," Arianna Huffington wrote in an email to Yahoo News. "His passion, his exuberance, his fearlessness. And above all, what I'm thinking of at the moment is his amazing wife Susie and their four beautiful young children." Matt Drudge wrote in a note to readers at the top of his website: "Andrew Breitbart was a constant source of energy, passion and commitment. We shared a love of headlines, a love of the news, an excitement about what's happening." The outpouring of condolences and reflections from conservatives--and even some liberals-- filled the airwaves and Twitter, as friends and colleagues applauded his courage and outspokenness. "He was one of the most fearless people I ever knew," a shaken Jonah Goldberg, founding editor of the National Review Online, said on Fox News. "One of his favorite pastimes was to retweet all of the hate that people threw at him, because he considered it a badge of honor. It was his Wheaties." "I'll remember his enormous, brawling passion and ready good cheer," ABC News' Terry Moran wrote on Twitter. "For better and worse, he reshaped modern journalism." Ned Ryun, president of the American Majority and one of Breitbart's friends, said: "He did something many in the conservative movement are afraid to do--go right at the left and not back down. He served as an example to the rest of the conservative movement of how to fight for our values without apology or compromise." In an update to his 2011 book, "Righteous Indignation," Breitbart emphasized his love of the chase."I love fighting for what I believe in. I love having fun while doing it ... I love fighting back, I love finding allies, and--famously--I enjoy making enemies." But in his final tweet, at 11:25 p.m. PST on Wednesday, the famously outspoken writer offered one follower an olive branch: "I called you a putz cause I thought you were being intentionally disingenuous," he wrote in response to Lamar White Jr., a Louisiana blogger. "If not I apologize." "While I never met Mr. Breitbart personally, I understand his last tweet was an earnest apology to me," White wrote in an email to Yahoo News. "Although I disagreed with him profoundly on politics and policy, I will always respect and admire his tenacious wit and his willingness to engage others in provocative conversation." Chris Moody contributed to this report.