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Borland Sidekick is a DOS based PIM (Personal Information Manager) and one of the first widely-used TSR (terminate and stay resident) programs. The key feature of Sidekick was that one could use Sidekick's utilities while using most other MS-DOS applications. This was important because MS-DOS had no built-in multi-tasking or task switching capabilities.
Stacker, from Stack Electronics, was a hard drive compression tool. It was wildly popular until Microsoft virtually eliminated the third party market for this by including their own drive compression tool with MS-DOS 6. and Expandz! Plus.