Time for the NL stories:
Atlanta: Bob Wickman (1.600)
Age: 37
Acquired: Trade, 7-06
This was a deadline deal; the Braves had tried several other guys, and had no success. Wickman had been a major-league closer since 1998, about as "proven" as you can get.
Florida: Joe Borowski (2.822)
Age: 35
Acquired: Free Agent, 1-06
For the second straight year, the Marlins signed a journeyman that ended up having success as a closer. Borowski flamed out with the Cubs in 2004 and 2005, signed with the Devil Rays, and had a modicum of success. Florida paid him just $327K in 2006.
New York: Billy Wagner (5.948)
Age: 34
Acquired: Free Agent, 11-05
Wagner was a marquee free agent in 2005, and the Mets gave him $43M over four years to be their closer. So far, he's lived up to the contract.
Philadelphia: Tom Gordon (3.394)
Age: 38
Acquired: Free Agent, 12-05
Gordon had been the Yankees' top setup man for two seasons, but wanted to close. The Phillies gave $18M over three season to do that.
Washington: Chad Cordero (3.880)
Age: 24
Acquired: 2003 Draft (1st Round)
Like Huston Street, Cordero was a closer in college, and made it to the majors extremely quickly. He became the Expos' closer in 2004, and held his job when the franchise moved to Washington. He only recorded 29 saves this season, but that's mainly because the Nats went 71-91.
Cincinnati: David Weathers (2.566)
Age: 36
Acquired: Free Agent, 12-04
The Reds tried a lot of relievers as closer in 2006, and gave up a lot of talent to revamp their relief corps during the season. Weathers garnered the most saves, and was decent.
Houston: Brad Lidge (0.800)
Age: 29
Acquired: 1998 Draft (1st Round)
Lidge didn't make the majors until age 25, and it wasn't until he made it to Houston did he become a reliever. He was the understudy to Billy Wagner and Octavio Dotel, and finally got his shot at closing in 2004. For two season he was one of the best in the business.
Milwaukee: Francisco Cordero (1.719)
Age: 31
Acquired: Trade, 7-06
Derrick Turnbow was a disaster for the Brewers, so they got Cordero in the Carlos Lee trade this past July. Francisco settled things down, and Milwaukee picked up his 2007 option.
Pittsburgh: Mike Gonzalez (3.490)
Age: 28
Acquired: 1997 Draft (30th Round);Trade, 7-03
Gonzalez has been a Pirate for all but 9 days of his career: he was sent to the Red Sox on the 22nd of July 2003 but was returned in a second trade on the 31st. He became Pittsburgh's full-time closer, and saved 24 games, quite a feat for such a bad team.
St. Louis: Jason Isringhausen (1.064)
Age: 33
Acquired: Free Agent, 12-01
Isringhausen was one of the Mets' hyped trio of pitching prospects in the mid-90s. After a couple injury-filled seasons in New York, he was dealt to Oakland, where he became their closer. St. Louis signed him after the 2001 season, and he's been a pretty steady stopper for them since.
Colorado: Brian Fuentes (2.179)
Age: 30
Acquired: Trade, 12-01
Fuentes arrived in Colorado from Seattle as part of the Jeff Cirillo deal. He spent a couple seasons with the Rockies in a setup role, then got a chance to close.
Los Angeles: Takashi Saito (5.468)
Age: 36
Acquired: Free Agent, 2-06
The Dodgers signed Saito to a minor-league free agent deal, and got much more than their money's worth. Saito appeared in 72 games, and struck out 107 in 78.1 innings pitched.
San Diego: Trevor Hoffman (5.995)
Age: 38
Acquired: Trade, 6-93
When Gary Sheffield wore out his welcome in San Diego (what else is new), the Florida Marlins sent over a 25-year-old Trevor Hoffman. 14 years later, Hoffman became the all-time saves leader with 482. And at age 38, he's still pretty good.
San Francisco: Armando Benitez (0.027)
Age: 33
Acquired: Free Agent, 12-04
So far, Benitez has taken home $14M from the Giants, and has pitched 68.1 innings and converted 36 saves. Not a great return on an investment.