CC65 is an excellent freeware C cross-compiler suite for Commodore/Atari/Apple machines.

Use this program to convert the C64 binary output file created by CC65 to a C64 PRG file
which can be read using a C64 machine or C64 emulator.

c64prg.c - 3k (Source code)
c64prg.exe - 7k (MSDOS executable)