There are many - if you narrow down the scope of your question we might be able to suggest some specific to your needs.
A notable interpreter is "Ch: A C/C++ Interpreter for Script Computing" detailed in Dr. Dobbs:
Ch is a complete C interpreter that
supports all language features and
standard libraries of the ISO C90
Standard, but extends C with many
high-level features such as string
type and computational arrays as
first-class objects.
Ch standard is freeware but not open source. Only Ch professional has the plotting capabilities and other features one might want.
I've never looked at this before, but having a c interpreter on hand sounds very useful, and something I will likely add to my toolset. Thanks for the question!
Edit:
Just found out that one of my favorite compilers, TCC, will execute C scripts:
It also handles C script files (just
add the shebang line
"#!/usr/local/bin/tcc -run" to the
first line of your C source code file
on Linux to have it executed directly.
TCC can read C source code from
standard input when '-' is used in
place of 'infile'. Example:
echo 'main(){puts("hello");}' | tcc -run -
