DR DOS 6.x
DR DOS is an MS-DOS compatible operating system from Digital Research that evolved from their earlier CP/M-86 based products Concurrent DOS and DOS Plus.
DR DOS was compatible with most PC/MS-DOS software, including Windows 3.1, disk utilities, debuggers, networking software, and TSRs.
Version 5 was especially successful as it beat Microsoft to the market with the ability to free up more of the 640k base memory, something that was badly needed at the time.
Later versions included even more powerful features, including a graphical shell called ViewMAX based on Digital Research GEM, disk caching, LapLink style file serial/parallel transfer, power management, built in help, an undelete tool, SuperStor disk compression, and a task switcher.
Over all, it was considered a very good product, and it appealed to power users.
DR DOS also played a key roll in giving computer manufactures leverage against Microsoft.
Release notes
Released shortly after MS-DOS 5.0, DR DOS added SuperStor drive compression, and TASKMAX task switcher, once again one-upping Microsoft.
Microsoft responded by placing the infamous "AARD" code in a December 1991 beta of Microsoft Windows 3.1, giving people the impression that DR-DOS might not be compatible with Windows.
Digital Research DR DOS 6.0 (1-16-1992) [Spanish] (3.5-1.44mb)
Digital Research DR DOS 6.0 (10-16-1991) [German] (5.25-360k)
Digital Research DR DOS 6.0 Manuals (1991)