Apparently someone ripped out the underlying DOS from Windows 98 and made it work as a standalone OS. (Unfortunately, the main site is down, but there's a good info & a download mirror here.)
Why, you ask? So you can keep using old hardware, but with better disk support, FAT32, drivers, etc.
Fun stuff. I've already installed it. I'm looking forward to silliness like running the Windows 3.1 shell on top of it!
Links:
Main info site (seems to be down): MS-DOS 7.10 Download
Main info site (google cache): MS-DOS 7.10 Download
Download Mirror: PeteWeb.com Forums - Old Operating System Links