Storyteller
I originally wrote this post a few months ago and am just now getting around to posting it up.
My brother and I got the "Storyteller" trophy in Batman: Arkham City this Labor Day, along with everyone else who got the game before Halloween 2011 and didn't cheat by setting their clocks ahead. Not that I have anything against people who do that (that would be crazy), but it's not something I do. I found it more fun to set reminders on Google Calendar for every major holiday of each month and play the game then. It also gave me an excuse to boot up Arkham City every once in a while.
The Storyteller trophy/achievement is where you, Batman, have to go into the bottom floor of the courthouse and speak with Calendar Man on specific holidays, resulting in him describing a crime he committed on that holiday in the past. The game uses the internal clock of whatever system you're playing on, so if you play the game on Christmas (or tell your system that it's Christmas by setting the clock manually instead of through the internet), Calendar Man will tell you a story about a crime he committed on Christmas. It's a cool Easter egg that gets you some good, violent stories and a trophy/achievement.
Arkham City isn't the first game to use a console's clock to trigger special events like this, but it's one of the best uses I've seen. I know that Animal Crossing does this to a much larger extent, too.
Did anyone else bother getting this achievement? Did you do it in real-time or did you change the clock? What other cool uses of this mechanic have you seen?