Simulated replay. This post describes a season played out with sports simulation software. Scores, standings and winners are the simulation's results, not the real historical record.
As the baseball season rounds the halfway mark, both leagues are in a bit of a fluid state this year. While the National League has a familiar face sitting atop the standings at the moment, two teams are nipping right on their heels. Meanwhile, the American League is being led by a team that very few people believed even had a chance to be competitive this season. Which teams are in front and on pace to face each other in the World Series? You can see the standings below.
American League:

For the first time since the American and National Leagues merged, the Chicago White Sox have put together a team that looks like it could be a winner. Despite an average offense, the White Sox have reached the top of the American League at the end of June with one of the better pitching staffs in all of baseball. Coming in right behind Chicago are the New York Highlanders, who still boast the best offense in the American League. The Washington Senators and Detroit Tigers are running a close race for third and fourth place, while the difference between the fifth place Philadelphia Athletics and the last place St. Louis Browns is just three and a half games. The biggest surprise this season in the American League is the showing by the Cleveland Naps, who are sitting at 32-37 and are well off the pace midway through the season.
For the White Sox, Tom Hughes has had the best first half of a season in his career. After winning 22 games in his first season in Chicago last year, Hughes currently has a record of 14-6, with an amazing 1.53 ERA and 86 strikeouts. Backing Hughes up on the Chicago mound is veteran Joe McGinnity, who came to the White Sox in an offseason trade with the New York Giants. So far this season, McGinnity has 12 wins, a 2.98 ERA and 53 strikeouts. His addition to the pitching staff seems to have given the White Sox the boost that they needed for a sustained run at the top of the league.
While Chicago's pitching staff is outstanding, their offense has left a bit to be desired. Frank Chance leads the club with a .260 batting average and 36 runs batted in, while Fred Clarke is right behind Chance, batting .258 with a team leading two home runs and 29 runs batted in. Where Chicago really shines is on the basepaths, where they have stolen an American League leading 110 bases through 72 games this year. Frank Chance is also pacing the Sox in that department, having stolen 24 bases on the season. If Chicago can keep up their great pitching with solid baserunning, they will be a difficult team to catch in the American League.

(L-R) Frank Chance, Fred Clarke, Joe McGinnity and Tom Hughes have fans on the South Side of Chicago thinking about the World Series for the first time.
National League:

Once again, the Pittsburgh Pirates find themselves leading the National League at the midway point of the year, but this year, their lead is very, very slim. Both the New York Giants and defending champion St. Louis Cardinals are just a half game behind the Pirates, while the Philadelphia Phillies sit just six and a half games back, with the potential to make an upwards move at any time. After the Phillies, the National League teams do drop off a bit, with the Cincinnati Reds and Chicago Cubs struggling along with mediocre seasons again. The Brooklyn Superbas aren't too far behind, but it looks like fans in Brooklyn are already looking towards the 1908 season for a chance at a pennant, while the Boston Doves are hoping to get some help for Ty Cobb in the upcoming amateur draft.
This season, Honus Wagner has staked his claim as the best player in baseball, and there are few out there that would attempt to challenge that claim. So far this season, Wagner is hitting .326, with five home runs and 58 runs batted in. All of those marks not only lead the Pirates, but also lead the National League. Wagner's five home runs are also more than the entire team total of the American League leading Chicago White Sox! Right now, it appears that nothing is going to stop Wagner on his quest not just to get the Pirates back to the World Series, but to also win his second straight Most Valuable Player award. Wagner isn't alone in his assault of National League pitchers this season. His teammate John Titus is also having an outstanding year, as he's batting .317 with 29 runs batted in and 48 runs scored.
On the pitcher's mound, the story with the Pirates once again begins and ends with Rube Waddell, who is having yet another excellent season. This year, Waddell has a midseason mark of 13 wins against four losses and an incredible 1.17 ERA. The amazing pitcher also leads the National League with 119 strikeouts in 162 innings of work and is on pace for a seventh straight National League Most Valuable Pitcher award. Fortunately for the Pirates, Waddell is having the season that he's having, because the rest of their pitching staff hasn't been up to snuff so far this year. Their next best pitcher, Frank Corridon, spent a good portion of the first half down in the minor leagues. Since he returned to the club, Corridon has done well, posting a 5-3 record in eight starts with a 1.42 ERA. Waddell will need the support of Corridon and the rest of the Pittsburgh pitching staff, because with two teams breathing down Pittsburgh's necks, they'll need all the help they can get to win the pennant this year.

(L-R) Frank Corridon, Honus Wagner, John Titus and Rube Waddell are putting together an excellent campaign for the first place Pittsburgh Pirates.