Redis

When your question gets answered very fast in stackoverflow, you have Redis to thanks for that. Ok, that's an oversimplification.


When you asked a question, its INSERT can be served very fast by SQL Server as there are no SELECTs needed be ran when other users are merely browsing for existing answers and questions, and that includes your currently written question too. Less database contentions, more questions gets easily served.


Sources:

http://highscalability.com/blog/2011/3/3/stack-overflow-architecture-update-now-at-95-million-page-vi.html

http://meta.stackoverflow.com/questions/69164/does-stack-overflow-use-caching-and-if-so-how

Salivating at C# 5's async/await + Redis combo: http://marcgravell.blogspot.com/2011/04/async-redis-await-booksleeve.html