The RARv3 archive format provides a high compression rate, supports split volumes, and other features. The only free software presently available for it is unrarlib, which can only read rar archives, not write them, and only up to rar v2.9, not the now-prevalent rar v3. Supporting it will take some effort, since the file format is not publicly documented , it will have to be reverse-engineered from rar files.
Ways to help. If you know of an existing project or you wish to spearhead the creation of such a project please contact campaigns@fsf.org letting us know the details.