This github repository is wel known among iOS developers around the world. If you are looking for open source libraries and frameworks for iOS - this is you starting point.
I also recommend to check out this list when you start to develop you own components and libraries because API and features of components that you find in available libraries can be nice example for you.
Also if you are looking for open source iOS project that needs contribution this list is great place.
After diving into it it is very hard to find what type of library is not written for iOS yet.