Abraham Scott, whose wife, Janice, died in the Pentagon, said he would have told her, "Baby, at least one perpetrator has been brought to justice".
Scott and others said they wanted other members of al-Qaeda brought to justice now that the Moussaoui trial is over.
Rosemary Dillard, whose husband, Eddie, also died in the Pentagon, said the American system of justice worked.
"It's not going to be what all the families want," Dillard said.
"It shows the world we're not going to stand for terrorists to come to our country and to be let loose."
Carie LeMack, whose mother, Judy Larocque, perished on American Airlines Flight 11, which crashed into the World Trade Centre, said her mother did not believe in the death penalty and would have been glad that he was sentenced to life.
"This man was an al-Qaeda wannabe who could never put together the 9/11 attacks," Lemack said.
"He's a wannabe who deserves to rot in jail."
George Bush, the US president, said the verdict "represents the end of this case but not an end to the fight against terror".