The Last Question by Isaac Asimov
The Last Question by Isaac Asimov. One of the most iconic SF stories ever, and yet I'm amazed how many people don't know it. It's not long, do read it.
I was reminded of it by Pascal blogging about 2012 (I first read about this a decade ago. I can't believe how famous it's becoming. It's the new millennium bug. But maybe this one will work), and mentioning another classic, Clarke's The Nine Billion Names of God. Also warmly recommended. (Spoiler warning, Pascal's blog post reveals the ending of the story.)
I love big-perspective stories, and stories which make you think.