I stumbled upon this today, while looking for a platform-independent library to handle zip files:
It appears to have quite a few more things that would be handy, like an XML DOM implementation, crypto routines (there must be hashing algos in there, right? I want those!), threading library and network programming.
I'll take a look on this, but it's possible that I will use it in a long-time project of mine: Nightfall, a free RTS game. See http://nightfall-rts.org/.
I'm currently using SDL and SDL_net for threading and networking, respectively, and I have my own tiny XML library (as I didn't want to drag in extra dependencies). SDL threading is not very advanced, and SDL_net doesn't support ipv6. The small XML library is quite limited.