There were many of these.

An impressive suite was the stuff from QuarterDeck. Their DOS extender was called QDPMI, and used the QEMM memory manager.

Quarterdeck DESQview, built on QEMM, truly extended DOS by turning it into a kind of multi-tasking system, allowing users to be more productive.

It was known for its high application compatibility.

I used DJGPP in the early 1990's, and the GO32.EXE extender, which did the job.