Some of my favorite answers:
NP vs NP-Complete vs NP-Hard -- what does it all mean?
Why do we need boxing and unboxing in C#?
Can anyone explain this algorithm for calculating large factorials?
What is memoization and how can I use it in Python?
Primitive cast and assignments in Java
mapreduce - how to anonymize a column's values
How does Java know where to jump to when breaking out of a loop?