skalibs is a package centralizing the free software / open source C development files used for building all software at skarnet.org: it contains essentially general-purpose libraries. You will need to install skalibs if you plan to build skarnet.org software. The point is that you won't have to download and compile big libraries, and care about portability issues, everytime you need to build a package: do it only once.
skalibs can also be used as a sound basic start for C development. There are a lot of general-purpose libraries out there; but if your main goal is to produce small and secure C code with a focus on system programming, skalibs might be for you.
skalibs is free software. It is available under the ISC license.
git clone git://git.skarnet.org/skalibs
Cross-compilation is tricky. skalibs provides system-agnostic interfaces, so all the other skarnet.org packages cross-compile easily; but some effort needs to be made to cross-compile skalibs, see this page.
Here are a few other libraries originating from the same place as skalibs, i.e. people start to reuse and package, or rewrite, code from Dan J. Bernstein, and then patch after patch, addition after addition, the code evolves into a project of its own: