ARJ

Overview
ARJ was one of the leading compression tools during the nineties. While it was a bit slower than PkZIP, it sported many more options, some of which were unique during that time (archives over multiple disks/volumes, finetuning of the compression algorithms used based on the data that was being compressed, recovery records to recover from simple transmission errors, etc.)

Resources

 * official homepage of ARJ software
 * Wikipedia page on ARJ
 * Page on fileformat.info
 * Open-Source implementation of an ARJ compressor/decompressor