I'm a little surprised to see /r/Patriots without the other four that I thought would be mentioned with it. So I'll pitch all five. Full disclosure: I'm a moderator of the Patriots team sub and of the NFL sub itself.
For those who don't follow the NFL, after today, there are four teams left in the post-season. Next Sunday, the New England Patriots will play the Indianapolis Colts and the Seattle Seahawks will play the Green Bay Packers. The winners of each game will meet two weeks later in the Super Bowl.
As a note, on reddit, the NFL subreddits tend towards civil football conversation, so if you're looking for somewhere to say something rude to fans of other teams, this isn't what you're looking for.
Relevant subs:
/r/nfl -- the big leagues itself. And no, you don't have to be an American to enjoy the sport or the subreddit; we'll happily explain anything that goes on.
/r/Patriots -- New England Patriots, mentioned above. If you don't follow the NFL, you might still know them as the team whose offense is lead by the husband of Gisele Bündchen. Or you might know them as the team whose coach was named by one of the Star Wars novelists to most likely be a Sith. Of all the teams remaining, they have the longest Super Bowl drought.
/r/Colts -- their quarterback looks like a fit version the reddit stereotype, neckbeard and all, although he's an incredible athlete. They travel next week to New England to play for the right to go to the Super Bowl. Just a few hours ago, they vanquished their former leader's new team in a decisive victory.
/r/greenbaypackers -- the only publicly owned team in the NFL has had one season each in playoff seeds 1-6 (there are only six per conference) in the past six years.
/r/seahawks -- the Seahawks are hosting the Packers next week to play for the right to go to the Super Bowl. The Seahawks also won last year's Super Bowl in a lopsided victory later nicknamed the "Orange Wedding" by many.
If you're already a fan of one of the 32 teams and didn't know about reddit's football forums, welcome! If you aren't and want to learn more, check it out.