Libcut (Lib rary for C U nit T esting) is an easy-to-use unit testing framework designed for C11. It's in early development, but it's very usable, consisting of a single header file (libcut.h). See the manual.
The output is nice and colorful, too; here's the output from running the tst.c program included:
FF error in test abc at tst.c:10: 1 != 0 error in test xyz at tst.c:5: 'a' != 'ab' Tests run: 2 Tests succeeded: 0 Tests failed: 2