A few weeks ago, Adele_L suggested that the repositories were underutilized and looked for suggestions on how to improve that. In that spirit, I added the following links to the Special Threads wiki page.

Solved Problems Repository - A collection of "solved problems in instrumental rationality."
Useful Concepts Repository - A collection of concepts that Less Wrong users have "found particularly useful for understanding the world."
Boring Advice Repository - A collection of advice that is optimized for helpfulness rather than depth of insight.
Useful Questions Repository - Questions that are useful to keep in mind in various situations.
Bad Concepts Repository - A collection of useless or harmful concepts
Grad Student Advice Repository - A collection of advice for graduate students.
Textbook Repository - The Best Textbooks on Every Subject
Reference repository - List of references and resources for LessWrong
Procedural Knowledge Gaps - How to do things that are "common sense" but that you may not know.
Mistakes Repository - A list of life-course altering mistakes that LW members have made.
Good things to have learned - A collection of skills and life lessons LWers have learned
Financial Effectiveness Repository - Tips for maximizing financial returns on (not necessarily market) investments.

In a similar vein, there is also a wiki page for the LessWrong Communities How-To's and Recommendations.

If there are other repositories that I've missed or a better way to collect these things, please link to it in a top level comment so that I get a direct message. A year and a half after this was originally posted, I still get suggestions and still add them or explain why I don't add them.

Edit: Added a few more to the list.