Since 2005, among players with at least 1,000 plate appearances, just four have a better on-base average than Nick Johnson. One of them is Barry Bonds.
Normally you couldn't trade for a hitter of this quality at any price, but Johnson will be a free agent this fall and he plays for the Washington Nationals, who are a punchline. Given the odds that the notoriously fragile first baseman will sprain something a day after being moved and make a prompt run to the 60-day disabled list, the Nationals won't be able to demand all that much in return for him. But that just makes him the player on the market who represents the greatest potential reward for the least actual risk.
Just consider that Texas Rangers first basemen have run up an appalling .275 OBA this year. Over two months, the difference between this and what Johnson usually does is within range of the improvement the Rangers could reasonably expect from replacing the worst member of their rotation with Roy Halladay. No other contender would benefit that much from adding Johnson, but the San Francisco Giants, whose first basemen have run up a .322 OBA this year, come close.
The antithesis of Johnson is probably Seattle Mariners starter/smoke- and-mirrors act Jarrod Washburn. Since June 1st, the 35-year-old has run up a 2.34 ERA. Given the eternal dearth of left-handed pitching in the majors, there will surely be some executive willing to overpay for his rights; that executive will be slapping himself on the forehead all winter long.
Whatever the supposed cause of Washburn's renaissance, the real explanation for his fine year is as simple as can be: As a left-hander pitching his home games at Safeco Field in front of an outfield with three true center fielders, he benefits from his environment as much as any player in baseball. He's given up a .259 batting average on balls in play this year; in the previous five years, that ranged between .289 and .309. Unless any potential suitors can finagle Safeco's righty-killing layout and Ichiro Suzuki's glove out of the Mariners as part of the deal, they'd be well advised to stay away.