Terms of the deal were not known.
Harrington, who lost the starting job to Chris Redman last season, his first with the Falcons, was released last week and re-signed to give the team additional salary-cap room.
The Falcons also cut veteran quarterback Byron Leftwich last month.
The 29-year-old Harrington played in 12 games with 10 starts last season. He completed 215 of 348 passes for 2,215 yards with seven touchdowns and eight interceptions. He set a career high by completing 61.8 percent of his passes, but the Falcons were only 3-7 in his starts.
Detroit drafted Harrington No. 3 overall in 2002.
Following five seasons with the Lions and one year in Miami, Harrington signed a two-year, $6 million contract to become Michael Vick's backup.
-- John Clayton, ESPN.com and The Associated Press
Safety Keith Davis has followed new Dolphins head coach Tony Sparano from Dallas to Miami.
Davis signed a two-year contract Tuesday. Terms were not disclosed.
Davis will also be reunited with Todd Bowles, the Dolphins' new assistant head coach and secondary coach, who was with the Dallas Cowboys the past three years.
Davis spent the past five seasons with the Cowboys and
was their special-teams captain. He made 69 tackles on special
teams in the past four years, and also has 22 career starts at
While with the Cowboys, Davis was a shooting victim twice before the start of training camp.
In June 2003, Davis was hit in the hip and elbow while picking up a friend at a topless club. He was then released on the first day of training camp. He rejoined the team the following season and became so valued that he started 15 games.
In 2006, one bullet grazed Davis' head and another lodged in his thigh when he was shot while driving on a Dallas highway. He was released from the hospital in good condition two days after the incident.
In Miami, Davis is expected to compete for a starting spot at strong safety. He also is expected to boost the team's special teams, which were a weak point during the Dolphins' 1-15 season in 2007.
He is the 10th player added to the Dolphins' roster since the free-agent period started on Feb. 29.
-- ESPN.com news services
Darling, Baltimore's third-round pick in 2004, played in 30 games in four seasons for the Ravens, catching 20 passes for 331 yards with three touchdowns. Eighteen of those catches came in 2007, when he played in every game.
In college, Darling played his freshman season at Florida State before transferring to Washington State after the death of his brother, Devaughn. Devaughn Darling died in 2001 at 18 of an apparent cardiac arrhythmia during an offseason workout at Florida State.
The brothers learned they had a genetic blood disorder, known as a sickle cell trait, from a physical before their freshman year at the school. Medical examiners said the disorder may have contributed to the death, although no definitive medical cause was cited.
The 35-year-old Kennison played for the Chiefs for seven seasons and was their top receiver for most of that time. He was limited by hamstring and shoulder injuries to only eight games last season, catching 13 passes for 101 yards.
-- The Associated Press
Detroit hopes the seven-year veteran can help fill the void left by trading Shaun Rogers to Cleveland.
Darby played for the Tampa Bay Buccaneers from 2001 to '04, when Detroit coach Rod Marinelli coached their defensive line. He was with the Seattle Seahawks the past three seasons, starting six games last season before going on injured reserve with a knee injury.
-- The Associated Press