I don't know how deliberate your choice of license was; if you haven't before, it might be worth considering making this available under LGPL instead of GPL.
Completely changes how you thing about strings!
Libowfat is a very useful tool in any C programmer's arsenal because it makes it very easy to build strings safely and securely without worrying about buffer overruns and hidden printf problems. Using it in conjunction with read(2) and write(2), its possible to avoid using the printf family of functions altogether!
It took me a little while to get used to this new paradigm, but I find it makes things a lot better.
An open, cross-platform journaling program.
A scientific plotting package.