DoubleDisk 2.5
Released in 1991 by Vertisoft Systems, Inc
For DOS

Image quality: Used

First released in 1989, DoubleDisk was the first real-time drive
compression product for DOS. It is a fairly no-frills program and
sticks to what it does best. It was relatively low cost compared to
its later competitors.

DoubleDisk works by creating a hidden file on the regular hard drive
that is then presented to the system as an extra drive letter. Files
copied in to this drive are invisibly  compressed and decompressed.
Applications do not have to take any special action or include any
specific support.

DoubleDisk is notable as its compression technology was licensed by
Microsoft for use in MS-DOS 6.0's DoubleSpace.

It competed with other compression tools such as Stacker, SuperStor,
and Expandz! Plus

Note: Version 2.5 is not backwards compatible with earlier versions.

2.5 was followed by DoubleDisk 2.6, then by an immediate jump to 
"DoubleDisk Gold 6.0". 6.0 Gold added better removable media support, 
faster performance, and support for EMS, XMS, and UMB memory, but was 
best suited to DOS 5.0 or earlier.