A killer app is software that’s so good it justifies using specific hardware or operating system. By analogy, a killer library is a library so good it justifies using a specific programming language. Two examples:
Usually there are alternatives to the killer library in other languages, but they don’t catch on for one reason or another— usability, lack of critical features, etc. This can change over time. Arguably quickcheck could have been a killer library for Haskell, but now most languages have a decent PBT framework.
What are some killer libraries for your language of choice?