There's an old joke that says all you need to have a long career in baseball is to pitch with your left hand and have a pulse.
It's a big-league exaggeration, of course, but the best lefties in the business are right on the pulse of what makes playoff contenders successful.
During this year's Hot Stove season, teams looking for lefty relievers are in luck. There is a small but strong group of southpaws available on the free-agent market.
Javier Lopez: Lopez is a nightmare to left-handed hitters. He held them to a .156 batting average and .431 on-base-plus-slugging (OPS) in 2013 and pitched to a career-low 1.83 ERA, with 37 strikeouts in 39 1/3 innings and a 1.06 WHIP. That could very well make him the headliner in this category and land him a nice multiyear deal, even at the age of 36.
Scott Downs: Downs kept doing what he does, putting up a 1.84 ERA for the Angels and a 3.38 ERA with the Braves, who acquired him two days before the July 31 Trade Deadline. Downs has always been stingy against lefties, and was again this year, holding opponents to a .630 OPS, but September struggles led to his absence from Atlanta's postseason roster and might cost him this winter.
J.P. Howell: Howell doesn't throw hard but he doesn't have to. His .164 opponents' batting average and .452 opponents' OPS against left-handed hitters in 2013 tells you all you need to know about his continued effectiveness. He's got playoff experience and a bulldog presence on the mound, and he isn't bad against righties, either, holding them to a .222 average this year.
Eric O'Flaherty: From 2010-12, O'Flaherty quietly blossomed into one of the premier left-handed relief pitchers, but Tommy John elbow reconstruction surgery cut short his 2013 season and makes the free agent a bit of a question mark. Teams willing to gamble on a quick return to effectiveness will look closely at O'Flaherty's .200 career batting average against and .531 OPS against left-handed hitters.
Manny Parra: Parra lowered his ERA from 5.06 in his first year as a full-time reliever in 2012 with Milwaukee to 3.33 in his first year with Cincinnati. Parra struck out 56 batters in 46 innings and only walked 15, which makes him a valuable commodity as he continues to adapt to his new role.
Boone Logan: Logan was one of the more durable and effective lefty relievers in the American League over the past four seasons with the Yankees. He comes off a season in which he posted the best WHIP (1.18) of his career and struck out 50 batters in 39 innings.
Oliver Perez: Perez still brings a fastball in the 94-95-mph range and is capable of stretches of dominant work now that he has reinvented himself as a big league reliever. But he's been inconsistent, which explains the brief rough patches that inflated his 2013 ERA to 3.74. Still, 74 strikeouts in 53 innings is no joke, and Perez should fetch plenty of interest.
Matt Thornton: Thornton is coming back from a right oblique injury that took him out of Boston's bullpen during the drive to the World Series. His strikeout numbers have dipped and his ERAs have risen as he's aged, but he's still an effective lefty-on-lefty option.
Michael Gonzalez: Gonzalez appeared in 75 games for the Brewers and struck out 60 batters in 50 innings. He struggled in the latter part of the season and will be 36 in May, but he still has good career numbers vs. left-handed hitters (.219 average, .634 OPS).
Rich Hill: Hill had 51 strikeouts in 38 2/3 innings but also walked 29 batters. That led to his overall 6.82 ERA, although he did limit lefties to a .238 batting average and .696 OPS.
Other possibilities: Travis Blackley, Tim Byrdak, Zach Duke, Pedro Feliciano, David Purcey.
Doug Miller is a reporter for MLB.com. Follow him on Twitter @DougMillerMLB.