I find it strange that no one is mentioning UWIN[1], started by David Korn about the same time and with the same goals as Cygwin. I used to use it. I liked the fact that it used the host compiler instead of gcc, plus I preferred the non-GNU tools. Now I'm using Inferno, it works much better. It's not Unix, it's Plan9, but I view that as a plus. Of course cygwin can install all kinds of Unixy tools, like gcc, openssh,…
(By the way, Inferno's implementation reminds me of Niklaus Wirth's Oberon, which could also run as either an application or as an operating system.)