Plumber Ian Puddick, 41, tweeted, blogged and posted videos online after being enraged by his spouse's 10-year relationship with City director Timothy Haynes.
Campaigners for free expression online celebrated at City of Westminster Magistrates' Court as District Judge Elizabeth Roscoe delivered a not guilty verdict after a three-day trial.
Puddick found out about his PA wife Leena's affair after reading a text message on her phone and discovering photographs of Mr Haynes performing sex acts in May 2009.
She met Mr Haynes after joining insurance firm Guy Carpenter in 1997. Their relationship gathered pace after they had sex after a Christmas party in 2002.
The affair continued until the May bank holiday in 2009 when Mr Puddick discovered a text message while he sunbathed in his garden in Enfield, north London.
The message spoke of a sex act and added: "You are such a sensual girl."
Mr Haynes was sending Mrs Puddick 30 to 40 text messages a day, some of them with graphic sexual detail.
They wined and dined together, with Mr Haynes commenting that she was "reassuringly expensive".
Mr Haynes, from Billingshurst, West Sussex, admitted he had been deceitful but said Puddick should have taken up his anger with him alone.
In tearful scenes at City of Westminster Magistrates Court, Mrs Puddick, 36, also claimed she suffered a miscarriage after Mr Haynes's wife, Annja, said she hoped her baby died.
But Mrs Haynes denied telling Mrs Puddick by phone: "I hope you never have children, I hope you never get pregnant and, if you do, I hope that baby dies."
Mrs Puddick had said Mr Haynes wanted to set her up in a flat as a "kept woman" after their extra-marital affair was exposed.
She also said she helped Mr Haynes, 52, fabricate his expenses to ensure that he was not wining and dining her out of his own pocket.
At one point he emailed her of his sadness that the relationship was over, writing: "Ignoring me is like the sun not rising."
Mr Haynes was attacked for his "extreme arrogance" during defence lawyer Michael Wolkind's closing speech.
In seeking to secretly continue the relationship after it was exposed, Mr Haynes illustrated how arrogant and deceitful he was, the court heard.
Mr Wolkind said there were many routes to an acquittal for Puddick.
"He is a man I present to you as a man whose evidence can be trusted completely," Mr Wolkind told the court.
"In contrast to Ian Puddick, Tim Haynes is a man whose evidence cannot be trusted at all.
"We know that 'Team Tim Haynes' wanted to deal with things in their own way. They wanted to smooth things over, they wanted to move on. Ian Puddick was not compelled to do the same."
Mr Puddick shook his fist and smiled amid cheers from the public gallery as two not guilty verdicts for internet harassment were returned.
After giving his wife a kiss, he said: "For the last 12 months this has taken over my life. Purely and simply there has been an abuse of power.
"It this can happen to me it can happen to anyone.
"It is absolutely a victory for free speech and the small man. I'm a plumber and drive around in a Transit."
He set up a string of websites and confronted Mr Haynes after finding out about the affair by reading a text message on her phone.
He also discovered photographs of Mr Haynes performing sex acts in May 2009.
Mr Haynes, from Billingshurst, West Sussex, admitted he had been deceitful but said Mr Puddick should have taken up his anger with him alone.
Mrs Puddick, who met Mr Haynes after joining insurance firm Guy Carpenter in 1997, said she was "looking forward to getting my husband back".
Mr Puddick added: "She's been on tea duty for the past two years."
The extra-marital relationship gathered pace after the colleagues had sex following a Christmas party in 2002.
The affair continued until the May bank holiday in 2009 when Mr Puddick discovered a text message while he sunbathed in his garden in Enfield, north London.
The message spoke of a sex act and added: "You are such a sensual girl."
Mr Haynes was sending Mrs Puddick 30 to 40 text messages a day, some of them with graphic sexual detail.
They wined and dined together, with Mr Haynes commenting that she was "reassuringly expensive".
In tearful scenes at City of Westminster Magistrates' Court, Mrs Puddick, 36, claimed she suffered a miscarriage after Mr Haynes's wife, Annja, said she hoped the baby died.
But Mrs Haynes denied telling Mrs Puddick by phone: "I hope you never have children, I hope you never get pregnant and, if you do, I hope that baby dies."
Mrs Puddick had said Mr Haynes wanted to set her up in a flat as a "kept woman" after their extra-marital affair was exposed.
She also said she helped Mr Haynes, 52, fabricate his expenses to ensure he was not wining and dining her out of his own pocket.
At one point he emailed her of his sadness that the relationship was over, writing: "Ignoring me is like the sun not rising."
Mr Haynes was attacked for his "extreme arrogance" during defence lawyer Michael Wolkind's closing speech.
In seeking to secretly continue the relationship after it was exposed, Mr Haynes illustrated how arrogant and deceitful he was, the court heard.
Mr Wolkind said there were many routes to an acquittal for Mr Puddick.
"He is a man I present to you as a man whose evidence can be trusted completely," Mr Wolkind told the court.
"In contrast to Ian Puddick, Tim Haynes is a man whose evidence cannot be trusted at all.
"We know that 'Team Tim Haynes' wanted to deal with things in their own way. They wanted to smooth things over, they wanted to move on. Ian Puddick was not compelled to do the same."
