Trade Market
Obviously, whoever gets Randy Johnson is goint to greatly improve their chances of winning the World Series. I think if either the Yanks or Sox could get him for prospects, they would be HUGE favorites. Even if the Sox have to give up Nomar, I think it makes them much better in the postseason (moreso than the regular season). Can you imagine a rotation of Johnson-Schilling-Pedro in the postseason? That would be almost unstoppable, particularly when you consider that none of the Yankees' pitchers are having particularly great years. Vazquez has been strong, but Mussina and Brown haven't pitched that well.
If the Angels can get the Unit, they're suddenly up there with the Yanks and Sox, and actually make winning the AL East a big deal, because the Wild Card will probably have to face the West winner.

There's another potentially big impact player who nobody seems to be talking about: Carlos Delgado. He's a free agent at the end of the year and the Jays are out of the race, so I imagine he could be had for a fairly low price. He's having a lousy year, but he's been one of the best hitters in baseball for years now (last year he led the AL in OPS+, and was in the top 10 the 3 previous seasons), so unless there's something really physically wrong with him, he should bounce back. He'd be a huge bat to add to any number of lineups.
I think the team that could really use him is the Giants. Batting Delgado behind Bonds would finally force teams to throw some strikes to Bonds, and if they don't, there would at least be someone to drive Bonds home.
The Angels could really use him, too, with the lousy production they're getting out of Erstad at 1B.
Or the Braves. Replacing LaRoche with Delgado might win the division for them.
Or the White Sox if Thomas ends up out for the season.
Or even the Yankees or Red Sox. The Yanks could DH him. The Sox could DH either him or Ortiz and move Millar into a platoon with Trot. And since the Yanks and Sox have all righty starters (for now), it would be extra useful to have that big lefty bat to hit against them in the inevitable ALCS.