Player pages now include a new section for platoon splits. You can see how each player did against lefties and righties, for any season or for their career. These splits are currently displayed for all seasons from 1974 to present, but may include earlier seasons as more data becomes available. Other split types, such as home/away, grass/turf, or by month are also planned.
A few interesting tidbits in the recent history of platoon splits since 1974:
Don Mattingly drove in the most runs in a season against lefties. He collected 66 RBI with a .288/.328/.568 line against southpaws in his AL MVP-winning 1985 season. Against right handers, the leader of the pack is Alex Rodriguez, with 131 in his 2007 AL MVP season.
The lefty-mashing HR mark is held by Cecil Fielder, who launched 25 bombs while putting together a .371/.479/.854 line in 1990. Barry Bonds's record 73 homers in 2001 also included a record 56 against right handed pitching.
The standard for most hits in a season against lefties is held by Lou Piniella, who touched them up for a remarkable 101 hits while hitting .349 in 1974. He only hit .249 against righties. The corresponding hit champ against righties is none other than Derek Jeter, with 184 hits and a .366 average against right handed hurlers in 1999. He added 35 hits and a .282 average against lefties. Ichiro Suzuki gets an honorable mention for having the 2nd, 3rd, and 4th most hits in a season against righties, with 181, 181, and 178.
What pitcher struck out the most right handed batters in a season? This one is easy. The top *seven* seasons are all by the Big Unit, Randy Johnson, topped by his 340 K's in 2001. Against lefties, Nolan Ryan's 207 K's in 1977 is the top mark.