Here is an AVL tree implementation, a library called libmavl.
You can download the source code as tarball:
ftp://ftp.planix.org/pub/planix/libmavl-1.0.0.tgz
Also you might want to run tests which requires to install libmutests you can download here:
ftp://ftp.planix.org/pub/planix/libmutests-1.2.0.tgz
You can browse the sour code via web:
https://websvn.planix.org/listing.php?repname=PlanixRsrch.SVN