Here is C language source code for an adaptive arithmetic coding library. I hope you find it useful. See the file ac.txt for more information.
ac.txt
ac.h
ac.c
acdemo.c

Some other sites that may be of interest:
The Arithmetic Coding Page (moved?)
Practical Huffman coding
comp.compression FAQ - source for arithmetic coding
ar.cod (the source code on which this package is based)
arith_coder (moved?)
dmc
arithshiftc.c