The concept that "I was acting rationally" isn't an excuse for predictably failing to maximize utility. I used to be a two-boxer on Newcomb's Problem; more practically, I believed that certain social situations were inherently biased against rational people.
I must not be a very good rationalist, because I find it blindingly obvious that if I believe one of two choices will have a better outcome, I should make that choice. :)
Since I participate in a lot of online discussions, three posts that are often in my mind are Politics is the Mind-Killer, Taboo Your Words, and Making Beliefs Pay Rent (in Anticipated Experiences). The taboo gives me a useful technique for eliminating confusion and semantic debates, and focusing on the actual difference between the models of reality held by the participants (found by looking at the different experiences anticipated by them). Knowledge of the mind-killer is harder to use, but I can at least refrain from introducing politics when I want a discussion to remain productive.
Here's my list:
Why Our Kind Can't Cooperate--I do a lot of collaborative work with groups of people who are "our kind". As someone who needs the reminder I reread this post frequently; it's open in an earlier tab right now. Actually, I need to share it with some other mailing lists...
This is some of the most amazing stuff on the whole web:
Can easily compete with Orion's Arm.
Two especially insightful posts on psychology that have become part of my brain by now are Wei Dai's The Nature of Offense and Yvain's Are wireheads happy? (this one was less than a year ago, but still).
Most of it, to be honest.
An Alien God is probably up there as the one that gave me the most to think about on a subject I thought I had more or less conceptually nailed.
The Least Convenient Possible World is probably the content that sees the most real-world action with me. Also the "crazy deity is playing a perverted game of blind poker with your eternal soul" line is probably the piece of LW dialogue I find myself wanting to reappropriate the most.
An Alien God I now regularly refer to evolutionary forces as Azathoth, it helps clarify the thinking. And it gives me a Power to fight against (a strong motivator for my psychology)
You Only Live Twice finally got me signed up for cryonics. As such, it has likely saved my life.
Yehuda Yudkowsky is still one of the most touching things I've ever read. I very rarely cry.
The Sword of Good got me to fundamentally re-order how I evaluate what is Good.
As someone who hasn't been around LW for a year yet, thanks for this! A list of memorable posts recommended by the regulars is much, much more appealing than the recommended technical background lists which already exist, and at least as useful.
For some information about what people found most insightful and useful in Eliezer's posts see The Most Important Thing You Learned and The Most Frequently Useful Thing.
What's still on your mind (in a positive way) a year or more after it was posted?