Recently I started working on a C API for ActiveMQ. The idea is to wrap the ActiveMQ-CPP library in a C accessible API in order to leverage all the hard work that has gone into making the C++ client. I've made some good progress so far, I've wrapped all the CMS API's for ConnectionFactory, Connection, Session, Producer and Consumer. I'm part way through mapping the CMS Message APIs into C APIs.
We still don't have any unit tests written for it but there's some sample code that compiles, and runs (at least it did).
It would be great if some C developers out there wanted to chip in and help out on this as my C skills are pretty rusty. There's still a lot to be done in the wrappers like better managing error conditions and reporting the errors in a meaningful way.
You can download the source from the Apache SVN here.