What is ES/2?
It doesn't exist yet. It's planned to be a new, from scratch, open source kernel for OS/2. Later it will expand to include its own drivers and everything else making it a truly open source OS/2.
The goal is it will be similar to what GNU did for UNIX, but GNU did it backwards. They wrote all of the drivers and support programs, and started on the kernel last (in the early 1990s). They still don't have their kernel completed, and used the Linux kernel instead.
ES/2 plans to make the kernel first, then write all of the drivers and support programs.
We'll see though. Lots of work to do.