In a video posted on YouTube, Madonna claps her hands and declares, "This is a historical evening! ... We are lucky to be sharing it with each other!"
Other celebrity supporters of Obama echo Madonna's good cheer.
Oprah Winfrey, who cried during the celebration in Chicago's Grant Park, told the TV show "The Insider": "I was so, so, so excited and then just sort of a calm came over me. It feels like it actually is kind of real, so it feels great."
Asked what this means for the country, she said, "It means democracy at its finest. It's just the best. We did this. America did this."
George Clooney said in a statement: "I congratulate President-elect Obama on his historic victory, and now it's time to begin unifying the country so we can take on the extraordinary challenges that this generation faces."
Actress Scarlett Johansson, who stumped for Obama throughout his campaign, said the "overwhelming hope that Obama inspires is infectious."
"I've always maintained a strong faith in the power of Obama's positive campaign and am so proud of every citizen who took a stand, participated in the political process and insisted their voices be heard," Johansson said in a statement.
Singer John Mellencamp reflected on the election on his Web site, saying he never thought he'd see the day when "a man of color could be president of the United States. But today I am so proud of America."
Singer Colbie Caillat, who cried when Obama won, said she thinks "our country is in safe hands."
Stars including Lindsay Lohan, Michael Moore and Kanye West posted messages online.
Reacting on her Myspace page, Lohan gushed, "OBAMA IS OUR PRESIDENT!" and said that everyone who voted, "no matter their choice ... should be proud for voting in the first place."
West, whose mother Donda died last November, posted a portrait of the president elect on his blog with the message: "HI MOM, OBAMA WON!"
Meanwhile, director-provocateur Moore responded on his Web site, saying, "Who among us is not at a loss for words? Tears pour out. Tears of joy. Tears of relief. A stunning, whopping landslide of hope in a time of deep despair."
Even more stunning: a concession from John McCain supporter Elisabeth Hasselbeck, who said Wednesday on TV's "The View" that she'll follow Obama's lead.
"I will jump in that line and support our president because that is what as an American I believe we should do," said Hasselbeck, who received a fist bump from co-host Whoopi Goldberg.