Not satisfied with what was available, I spent the day writing a compact, platform-independent library for working with UTF-8, UTF-16, and UTF-32. This may be of some use to other people here, so I thought I'd share.

Header: UTFUtilities.h
Implementation: UTFUtilities.c

The code has passed some rigorous unit tests, but if anyone does find any bugs, please let me know. Enjoy!

This is amazingly something I need just now! It's going to be the finishing touch for my ATSUI -> Alpha Texture module. Thanks!

Only comment:
License: BSD that code if you want people to freely use it. It's copyrighted by default.

Aha, license is a good idea. I like zlib/libpng, so I went with that. Reuploaded with license comments included.