GeekDOS is a free and usable DOS variant based on FreeDOS, with a built in GUI and many small utilities. It can be installed to a hard disk or completely ran off of a CD/DVD or USB drive.
GeekDOS runs on very old hardware, and is intended to replace early versions of windows on very old machines. Minimum requirements are: a i386 or better intel/AMD cpu, 16MB of RAM for the CD version, and 640k for the USB, but I will be surprised if anyone finds a USB boot capable intel with such a low amount of RAM. A hard disk is not required unless you wish to install it to your system, in which case you will need a fat12, fat16 or fat32 partition with a little more than 10 mb free, but GeekDOS can be run just fine without being installed, from the media, of course. A hard disk is not needed. Changes will not be saved on the CD-ROM version unless installed, it must copy itself to RAM in order to boot properly from a read-only medium like a CD. On the properly written USB version, all changes will be saved to the USB device. GeekDOS is capable of simply installing the GeekDOS apps and leaving you with your current OS.
Download: GeekDOS 15.05