Ask HN: Any lib ideas?

One of my favourite parts of programming is creating reusable self-contained libraries. It is much more enjoyable for me to tweak the performace or the API of a library with a clear and simple goal than writing the often obscure 'business logic' of big applications. On the other hand I think the 'lib market' is quite saturated. Currently I do not have a good idea about what kind of lib I should write. Do you have ideas about some holes in the 'lib market', where the current libraries are lacking in performance, features or simplicity of their API? What kind of lib would you need? Do you think one can make a living off of writing libraries (reuasble tech-heavy code), or is it always expected that a library should be free?