The valuable September call-up is a rarer creature than you might guess. Oh, plenty of great players have been September call-ups -- Ryne Sandberg, Mike Piazza, Gary Sheffield, John Olerud, Paul O'Neill, Alan Trammell and Lou Whitaker on the same day -- but most of these guys didn't actually play much, and none of them were key parts of pennant pushes.
Essentially, if management thinks a player can make a real difference in September, they'll usually summon him from the minor leagues before Sept. 1, because they know he'll be eligible for postseason play, and anyway, why wait until the rosters expand?
I went back to 1970 and found only three players who 1) debuted between Sept. 1 and Sept. 10, 2) earned significant playing time, and 3) played a big role for a team that earned, or nearly earned, a postseason berth. (And since three players aren't enough for a whole column, below you'll also find a couple of players who don't quite meet those criteria.)