Unlike baseball's planned premier events, such as the World Series or the All-Star Game, the trade deadline wasn't envisioned as one of baseball's most significant seasonal moments.
Baseball's first MLB-wide trade deadline was instituted for the 1923 season, to standardize the differing deadlines that the American League and National League had instituted. Simultaneously the result of a war between commissioner Judge Kenesaw Mountain Landis and AL president Ban Johnson and between Cardinals owner Branch Rickey and the owners of the New York teams (among other feuds), the June 15 trade deadline stayed in force for generations. But it was never a Very Big Moment in baseball's season ... until 1986, when baseball moved to a July 31 deadline, which stayed in effect until the very-recent-but-slight change to Aug. 1.
As it turned out, July 31 turned out to be a fairly elegant date for the non-waiver trade deadline; it was late enough in the season that more teams are throwing in the towel than in mid-June, but late enough that players can have a real effect on the pennant races. Combine that with free agency -- meaning teams falling from contention would frequently have "move 'em or lose 'em" players -- and you have one of baseball's most exciting in-season times.
As might be expected, with only two months of playing time for a player acquired right at the deadline, the list of best trade adds isn't just made up of superstars. Some decidedly non-superstars turned out to be key pennant race adds too. Before we get to the projections for this year's best realistic trade additions at the deadline -- there is no Mike Trout projection listed below -- let's look at some of the most valuable adds of the July deadline era, ranked by FanGraphs WAR (fWAR) and listed with the team that acquired him.