Most are only incidentally C related.
- Several blogs at Embedded Gurus
- Embedded in Academia Atomic
- Object's Embedded Software and Embedded Corner
- Timo
- Object Mentor has some good entries on C
- Electronic Design Automation Blog
EDIT: updated to add more
Straight C programmers aren't particularly noted as a community that loves things like blogging. That said, the comp.lang.c usenet newsgroup is where I learned all sorts of interesting things about C. They are pretty hard core in there, and reading the archives will teach almost anybody quite a lot about the language, and where the language ends and your platform and libraries begin. There is also a comp.lang.c.moderated group, but it has generally had less traffic. Both are full of useful info, though.
There's really only one that I know and follow regularly. Raymond Chen's The Old New Thing. Cranky SOB, has been around at MSFT for a long time, before Windows 95. Well past the typical survival years for a Microsoft engineer. Deeply C at heart, with a wink at C++ when necessary. Firmly un-dot-netty, the sub-title of his blog.
He keeps his blog very fresh by drawing in war stories from his experience that are relevant to any programmer, no matter the language. Often Windows related, but that's just a vehicle to tell his stories. Very low threshold for the crap and not shy to expose it.
Recommended.