It's about time we talked playoff scenarios!
At present, five of the eight playoff spots have been claimed:
AL East Champion: Tampa Bay
AL West Champion: LA Angels
AL Wild Card: Boston
NL Central Champion: Chicago Cubs
NL West Champion: LA Dodgers
The Cubs have clinched home field advantage until the World Series, but the Angels have yet to do so. The only way they will lose that right, though, is if they lose their last three games and the Devil Rays win their last three. Based on the head-to-head tiebreaker, Tampa Bay would have home field advantage throughout the playoffs.
With three games left, Tampa Bay leads Boston by two. If they wind up tied, Boston will be the AL East champion thanks to the head-to-head tiebreak, and Tampa will enter as the wild card.
Should Minnesota and the White Sox tie, on the other hand, they will have a one-game playoff to decide who moves on and who goes home - any ties that would eliminate a team must be decided by playoff games. Home field advantage for that playoff would be determined by a coin flip.
If the LA Angels are the AL's top-ranked team, they will play the wild card winner (either Boston or Tampa), leaving the division champion to face Minnesota or the Chicago Sox. Ironically, that scenario holds even if Tampa pulls out a comeback on the Angels in these last three games - they can't play the wild card if it's from their own division.
To the NL! If Milwaukee wins the wild card, the Cubs will face the Dodgers and the Brewers will play the NL East winner (Philadelphia/NY Mets). Should either the Phillies or the Mets win the wild card, Chicago would play that team and the Dodgers would square off against the NL East winner.
If the Phillies and Mets end their season in a tie, and both of them have better records than Milwaukee, the Mets will win the division based on the head-to-head tiebreak.
If the Phils, Mets, and Brewers are in a three-way tie at the end of the season, the former two teams will play first to determine the division champion. The loser will then play the Brewers to decide the wild card.
The Astros are still in it, too. If they get into a three-way tie with the Brewers and Mets, the three will be designated teams "A," "B," and "C." The Astros will have the right to choose their designation first, since they have the highest winning percentage between the three teams. (Earlier tiebreaks are omitted since Milwaukee was better in head-to-head matchups against Houston, Houston was better than the Mets, and New York was better than the Brewers.) The Brewers will choose second, and the Mets third. Team "A" will then host team "B," and the winner will play at home against team "C."
Don't like reading so much text? Let me summarize it in statistical form.
(I'll omit the obvious ones... all ties that would eliminate a team have to be decided with a playoff game, and teams can't play the division series against a divisional rival.)
AMERICAN LEAGUE SCENARIOS
Minnesota > Chicago Sox
#1 LA Angels vs. #4 Boston
#2 Tampa Bay vs. #3 Minnesota
Chicago Sox > Minnesota
#1 LA Angels vs. #4 Boston
#2 Tampa Bay vs. #3 Chicago Sox
NATIONAL LEAGUE SCENARIOS
Philadelphia = NY Mets, Philadelphia & NY Mets > Milwaukee
#1 Chicago Cubs vs. #4 Philadelphia
#2 NY Mets vs. #3 LA Dodgers
Philadelphia = NY Mets = Milwaukee
Playoff #1 (NL East Champion): Philadelphia vs. NY Mets
Playoff #2 (NL Wild Card): Playoff #1 Loser vs. Milwaukee
Philadelphia > NY Mets, NY Mets > Milwaukee
#1 Chicago Cubs vs. #4 NY Mets
#2 Philadelphia vs. #3 LA Dodgers
NY Mets > Philadelphia, Philadelphia > Milwaukee
#1 Chicago Cubs vs. #4 Philadelphia
#2 NY Mets vs. #3 LA Dodgers
Philadelphia > NY Mets, Milwaukee > NY Mets
#1 Chicago Cubs vs. #4 LA Dodgers
#2 Philadelphia vs. #3 Milwaukee
NY Mets > Philadelphia, Milwaukee > NY Mets
#1 Chicago Cubs vs. #4 LA Dodgers
#2 NY Mets vs. #3 Milwaukee
If the regular season ended today...
AMERICAN LEAGUE
#1 LA Angels vs. #4 Boston
#2 Tampa Bay vs. #3 Minnesota
NATIONAL LEAGUE
#1 Chicago Cubs vs. #4 LA Dodgers
#2 Philadelphia vs. #3 Milwaukee
...Any questions?
(Thanks to Baseball-Reference.com.)
EDIT: Whoops, I forgot that the Astros were still mathematically alive.
EDIT2: Scenarios edited to reflect the most recent results.