Oh yeah, there's all kinds of C stuff out there.
There's an enlarged Reference page
Try some of the links at the lower right of the page too:
"Looking for something else? See the libraries page for interfacing with
particular types of hardware. Try the list of community-contributed code.
The Arduino language is based on C/C++. It links against AVR Libc and allows
the use of any of its functions; see its user manual for details. "