A post on the site read: "Media Matters has a long history with Andrew Breitbart. We've disagreed more than we've found common ground, but there was never any question of Andrew's passion for and commitment to what he believed. Our thoughts and prayers are with his family today."
Sherrod, who sued him for reputation damage last year, also released a statement. She said: "The news of Mr Breitbart's death came as a surprise to me when I was informed of it this morning. My prayers go out to Mr Breitbart's family as they cope through this very difficult time."
A former Georgia state rural development director, Sherrod, who is black, was fired from her job in July 2010 after the appearance of an edited video of her making what appeared to be a racist remark.
She is seen telling a local NAACP group that she was initially reluctant to help a white farmer save his farm more than two decades ago, long before she worked for US government.
Missing from the video was the rest of the speech, intended as a message of racial healing, in which Sherrod said he realised her mistake and helped the farmer.
Sherrod sued Breitbart, his employee, Larry O'Connor, and an unnamed "John Doe" defendant for "defamation, false light and intentional infliction of emotional distress".
When the lawsuit was filed against him, Breitbart said she was fired not because of the film but due to her part in the 11-year-old Pigford case, in which black farmers sued for discrimination against the agriculture department.
Breitbart, an outspoken critic of the mainstream media, said: "I can promise you this: neither I, nor my journalistic websites, will or can be silenced by the institutional left, which is obviously funding this lawsuit."
A lawyer for O'Connor told the Associated Press on Thursday that it is unclear whether the case will proceed for the other two defendants, who were in the process of trying to get the case dismissed from federal court.
It was not the first time he triggered a storm of criticism.
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