Meta: Send this to anyone who is interested in learning more about "rationality"
A refresher: what is “rationality?” 
Rationality is the art of thinking in ways that result in accurate beliefs and good decisions [as understood by the LessWrong community; this understanding of rationality differs from others, some of which are more common in colloquial usage than “LessWrong rationality”]. It is the primary topic of LessWrong.
Rationality is not only about avoiding the vices of self-deception and obfuscation (the failure to communicate clearly), but also about the virtue of curiosity, seeing the world more clearly than before, and achieving things previously unreachable to you. The study of rationality on LessWrong includes a theoretical understanding of ideal cognitive algorithms, as well as building a practice that uses these idealized algorithms to inform heuristics, habits, and techniques, to successfully reason and make decisions in the real world.
Resources and materials
To learn more about rationality, I recommend (in order of usefulness per unit of effort):
- Subscribing to the LessWrong digest (subscribe at the sidebar),
- Reading the essays from the list below,
- Reading Rationality A to Z (podcast form here),
- Reading some of the best works from Scott Alexander.
To learn more about systematically doing good (i.e. effective altruism) I recommend:
- Subscribing to the 80,000 Hours mailing list,
- Reading this 80,000 Hours post about the importance of career choice,
- Participating in an EA Virtual Program,
- Reading/skimming this comprehensive set of readings on effective altruism.
Some of the best rationality essays 
Below you can find some of the best writings on rationality, in no particular order:
- Affective Death Spirals
- Uncritical Supercriticality
- Cached Thoughts
- We Change Our Minds Less Often Than We Think
- Hold Off On Proposing Solutions
- Knowing About Biases Can Hurt People
- Update Yourself Incrementally
- The Bottom Line
- Avoiding Your Belief's Real Weak Points
- Motivated Stopping and Motivated Continuation
- The Proper Use of Humility
- You Can Face Reality
- The Meditation on Curiosity
- Something to Protect
- Crisis of Faith
- Mind Projection Fallacy
- Explaining vs. Explaining Away
- Angry Atoms
- Making Beliefs Pay Rent (in Anticipated Experiences)
- Humans are not automatically strategic
- Local Validity as a Key to Sanity and Civilization
- Twelve Virtues of Rationality
- The noncentral fallacy - the worst argument in the world?
- Policy Debates Should Not Appear One-Sided
- "Other people are wrong" vs "I am right"
- On Caring
- Outline of Galef's "Scout Mindset"
- Generalizing From One Example
- Diseased thinking: dissolving questions about disease
- How An Algorithm Feels From Inside
- Scope Insensitivity
- Conservation of Expected Evidence
- Your Strength as a Rationalist
- Your intuitions are not magic
- How To Convince Me that 2 + 2 = 3
- The simple truth
- Circular altruism
 Taken from the Rationality Tag, though the bracketed words are mine.
 Half of these are the bolded essays from here, the other half were recommended by a well-read LessWronger. By “some of the best” I really mean “some of the most useful to read, probably.” This is not authoritative at all; please comment if you think we missed any important essays.