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The CCS http://www.ccsinfo.com/picc.shtml compiler is a really popular choice and probably the most user friendly since it does all variable allocation itself (like a good compiler should). Other compilers tend to require the programmer to specify locations for variables, etc, manually.
After working with several COMPILERS : CCS, C2C, CC5X .....
I would recommand Hitech for serious developpement.
Integration in MPLAB6.1 is now very good.
Free version (piclite) is more and more interesting.
But the intial post says INTERPRETER.
If this term is intentional, I think the answer is :
"no C interpreter on Pic !"