I've started using Check, a unittesting framework for plain old C. The documentation isn't lacking but here's an example test file and the corresponding Check unittest Makefile anyway.

Check can also output XML, which is useful for reporting on nightly builds. I've created an XSLT to convert to HTML: check unittest.xslt. It's very basic -- adjust to your needs. You can generate such a report on the commandline with a quick

  $ xsltproc check_unittest.xslt check_output.xml > report.html