On behalf of America, specific to countries.
To Canada and Russia: Sorry for Alaska and the monster in our political system from it, If it were up to me I'd let you two annex it, especially since it's full of oil and we're teasing you by not even trying to drill it.
To all socialist/communist states: I'm sorry we try to say we're better than you and go out of our way to be mean to you, you're government works for you and ours for us, or at least capitalism used to love us but I think it changed it's mind in recent years.
To Britain: Sorry for taking credit in wars that we couldn't have won without you. Also thanks for the good bands that you sent to us (ei, the beatles, pink floyd, rolling stones, etc. there's a lot of them and they were all awesome). Also sorry for not speaking your language correctly.
To the world, but mostly Samoa: Sorry for our disgusting fast food that we've poisoned ourselves and everyone else with, although I always appreciate that as long as we have mcdonalds I could hit rock bottom and still be able to get a square meal for a dollar.
To the world, but mostly Australia: Sorry for our tourists.
To a number of developing nations: Sorry for messing up everything by funding civil wars, especially if it was just to use you for resources or to beat on some communists, sorry for conflict diamonds in Africa, sorry we won't leave the Middle East. really sorry for everything.
To Russia and others who still consider yourselves Soviets: Sorry we still fantasize about being heroes in wars against you.
To Muslims across the world: Sorry we're so ignorant and prejudiced to your religion; 911 hurt but that doesn't justify "burn a koran day"
To gays across the world: Sorry for a number of things, but most recently taking so long to repeal don't ask don't tell
To Europe in general, but mostly France, England, Ireland, Germany, and Russia: Sorry for our horrible jokes about you.
To Mexico and the rest of Latin America: Sorry if we let those who've immigrated here down with that whole "American dream" ideal that no longer exists, and sorry for the prejudice and our immigration policies.
To China: Sorry for being hypocrits by hating on your government and talking about how underpaid sweatshop workers make everything at walmart but still buying your stuff.
To North Korea (not that you can read this unless you've escaped): Well really just to the government, but sorry in advance for how bad we're gonna fuck you up and sorry to everyone for not having done so already.
To Japan: Sorry for stereotypes, although I still find it sad that you're taking over our pop culture, I miss good American animated cartoons...
To Everyone everywhere ever: Sorry for our arrogance and truly awful TV.
To any aliens: Sorry for the conspiracies about you and the fact that we like to fill space with radio gargles just so you'll hear us, although I'll never apologize for the show invader zim, which as far as I'm concerned is the greatest show in existence.
I think that's most of it. Jeeze we're messed up.