FreeDOS is an effort to create a free DOS Operating System, which is compatible with MS-DOS applications and drivers, yet have some features that are missing in MS-DOS. The software is currently in Beta status, but the release of the first official version - FreeDOS 1.0 has been planned. Presently, some FreeDOS commands are identical to or better than their MS-DOS counterparts. However, a few MS-DOS commands and functions are still missing or have bugs in FreeDOS. The command line interpreter used by FreeDOS is called FreeCOM.
- Open source/Free license (GPL)
- User-editable internationalization support
- FDAMP - APM control/info, energy saving TSR/control, cache flush, rebooting, and ASPI support
- Ultra DMA driver and support for disks larger than 127GB (LBA) (can also be used in MS-DOS)
- LBACACHE - disc cache (SMARTDRV in MS-DOS)
- FDCDEX and CD-ROM driver (MSCDEX in MS-DOS)
- CTMOUSE - Mouse driver with wheel-support (can also be used in MS-DOS)
Note that FreeDOS supports the FAT32 file system (except for a few FreeDOS commands, such as DEFRAG), whereas MS-DOS versions prior to 7.1 do not. Also, Long File Names (LFN) APIs (which can be provided by the DOSLFN driver or similar) and large disks (LBA) are not supported by MS-DOS 6.x (or earlier), and most of the FreeDOS programs. 
Windows 1.0 through Windows 3.x/3.xxEdit
FreeDOS seems to work fine with 1.0 and 2.0 versions of Windows. 3.x versions are tricky to run; the simplest command that currently works for version 3.1 is WIN /S /D:SFVX (S: no breakpoints in ROM, F: no 32 bit disk access, V: use BIOS int 13 to access disk, X: do not touch UMB area).
To date Windows 3.11 has not been successfully started under FreeDOS.
If you manage to use WIN /3 (386 Enhanced Mode) or Win32s, you need to contact the FreeDOS people and spread the word about which configuration you used to do that.
Windows 95, 98 & MEEdit
Those Windows versions contain and are linked to their own DOS (MS-DOS). You cannot run them in FreeDOS, but you can install Windows and FreeDOS on the same C: drive, with help of a boot manager as described above or a boot manager like LILO or GRUB.
Windows NT 4.0, 2000 and XPEdit
A parallel installation with Windows NT 4.0/2000/XP doesn't cause problems. because these versions don't use a DOS system as a base operating system, and they are shipped with a simple boot manager. The FreeDOS kernel can simply be added to Windows' boot.ini to enable its selection in the boot manager (by adding a boot sector file or drive to the list - FreeDOS will always search the first FAT16 or FAT32 drive for fdconfig.sys or config.sys, even if you boot it from another harddisk drive letter
The EMM386 memory management program included with FreeDOS now supports VCPI, which allows programs and DOS extenders which use DPMI to run. FreeDOS also contains an UDMA driver for faster disk access, which can be used for other DOS versions, too. The LBACache disk cache buffers recently-accessed disk data in XMS memory for even faster access and less direct access (reduced noise) to the harddisk.
With help of the ability of the FreeCOM command line shell to swap itself to XMS, it is possible to get much of the lower DOS RAM free: With kernel and BUFFERs in HMA and drivers (where compatible with this) loaded to UMBs, 620k (620*1024 bytes) and more can be available. This can be useful for memory-hungry DOS games. 
Some advantages of FreeDOS: The license is free and the software is actively being developed. FAT32 support is implemented and it is possible to boot from FAT32 drives using FreeDOS. Depending on the BIOS used, LBA hard disks up to 128GB or even 2 TB in size are supported. Some BIOSes support LBA but have a bug for disks [[bigger than 32GB. You can use a driver like OnTrack or EzDrive to "repair" that problem. FreeDOS can also be used with a driver called DOSLFN which supports long file names (see VFAT), but most FreeDOS programs do NOT support long file names even if the driver is loaded.
There is no planned support for NTFS or Ext2, but there are several shareware drivers available for that purpose. To access Ext2, you can use the LTOOLS (counterpart to MTOOLS) which can copy data to and from Ext2 drives.
If FreeDOS is started from a DOSEmu (PC/DOS emulator for Linux systems) window it is possible to install DOS applications on any Linux supported file system and hard disk. Also there is no USB driver support planned, in many cases only BIOS supported USB devices are available for plain FreeDOS. You can also try some DOS USB drivers (such as USBASPI and USBMASS drivers, although they are mainly designed for use with MS-DOS) for other USB storage devices, or run FreeDOS in a DOSEmu window and let it use any drive which can be accessed from Linux that way. Other popular PC emulators and DOS emulators are Bochs (simulates a whole PC) and DOSBox, which simulates a PC with a DOS kernel and shell simulation: Programs inside DOSBox "see" a DOS, but you cannot install a FreeDOS or other DOS kernel. You can, however, use FreeDOS tools inside DOSBox.
The FreeDOS kernel is also shipped with DOSEmu. DOSEmu supplies a DOS optimized simulation of a PC, which allows the use of simplified drivers (shipped with DOSEmu). The System runs much faster than the GNU PC simulator Bochs or the commercial emulator VMware. However, the simulation of raw hardware lacks realism in some aspects: Simulated disk access through the virtual BIOS works fine, but DOS programs cannot program the virtual disk controllers. There is virtual graphic and sound hardware, though. 
As a result of a license agreement with Microsoft, which obligated computer manufacturers not to ship computers without Operating Systems, Dell Computers offered some of their n-series systems with pre-installed FreeDOS.
One alternative to FreeDOS is OpenDOS. This DOS is more compatible with Windows, but the license is less free. OpenDOS is a derivative variant of DR-DOS, which is owned by DeviceLogics and offered under shareware-like terms. DR-DOS offers a good memory manager (EMM386, here with a built-in DPMI host and multitasking support and XMS/EMS memory pool sharing) and real preemptible multitasking.