This covers plain DOS; for Windows or OS/2 programs, see the next sections.

NOTE ABOUT SIMTEL FILES: The best-known Internet collection of PC-related
programs is the Simtel archives (named for the original archive site, now
defunct).  The principal archive site for these files is ftp.simtel.net,
which is the site referenced by the FTP pointers given below.  However,
there are numerous mirror sites that keep copies of the Simtel files.
For quickest response you should use the mirror site closest to you.
Consult the periodic postings in comp.archives.msdos.announce to find your
nearest mirror site.  If you have no FTP capability, the same postings will
tell you how to retrieve Simtel files by e-mail.  You can also access the
Simtel archives via WWW at www.simtel.net.

QPV (formerly called QPEG) is an extremely fast JPEG viewer. In exchange for
speed, QPV gives up some image quality, particularly on 256-or-less-color
displays.  Its best feature is a really-fast small preview window, which is
great for searching through lots of image files. Also views GIF,TGA,BMP,PNG.
Requires 386-or-better CPU and VGA-or-better display card.  Current version
is 1.7e, from ftp.tu-clausthal.de:/pub/msdos/graphics/view/qpv17e.zip.
Shareware, $20.

SEA is a new JPEG/PNG/GIF/etc viewer and file-format converter.  It is
very very fast --- faster than QPV in most cases, according to the authors.
Also, it can read progressive JPEGs; QPV can't.  Current version is 1.3,
available from ftp.simtel.net:/pub/simtelnet/msdos/graphics/sea3.zip.
Shareware, $30.  Requires 386-or-better CPU and VESA-compatible display.

DVPEG is a free viewer for JPEG, GIF, Targa, and PPM files.  Current version
is 3.0l, available from ftp://sunee.uwaterloo.ca/pub/jpeg/viewers/.
(That's lower case l, not digit 1.)  This is a good basic viewer that comes
in both 286 and 386-and-up versions.  The user interface is clunky but
functional.  DVPEG is substantially faster than it used to be; on hi-color
displays it is nearly as fast as QPV.  On 8-bit displays, its two-pass
quantization mode is slow but gives much better image quality than QPV can
provide.

Lesser-used DOS viewers include:
* DISPLAY, alias DISP.  The Swiss army knife of DOS viewers.  Does almost
  everything, but a bit intimidating for newcomers.  User interface is much
  improved over early versions, but still awkward in places.  Requires 386
  or better.  Freeware.  Current version is 1.89, available from
  ftp.simtel.net:/pub/simtelnet/msdos/graphics/disp189a.zip and disp189b.zip.
* GDS.  A well-done viewer and image converter for many image formats.
  Installation is simple, and the on-line documentation is very good.
  JPEG loading is a bit slower than the above viewers, though.  Shareware,
  $40.  Current version is 3.1f.  A slightly restricted demo version is
  available from ftp://ftp.photodex.com/.
* NVIEW.  Views JPEG and half a dozen other image formats.  Easy to use,
  very easy to install.  Only moderately fast, but it has lots of options.
  Supports hi-color and true-color modes on some cards, but not mine :-(.
  Requires 386 or better.  Current version is 1.50, available from
  ftp.simtel.net:/pub/simtelnet/msdos/graphics/nview150.zip. Shareware, $29.
* CSHOW or CompuShow (recently renamed 2SHOW).  This is a widely used viewer
  for GIF and other formats.  Versions prior to CSHOW 9.00 or 2SHOW 2.00 had
  absolutely abysmal JPEG support; if you have one of those, toss it and get
  a newer version.  The current release is still the slowest DOS JPEG viewer
  listed here, but it's faster than it used to be, and image quality and
  robustness have improved substantially.  The main reason to use CSHOW
  as a JPEG viewer is that it supports a wide range of pre-VGA display
  hardware (most of the above viewers require VGA or better).  Also, CSHOW
  doesn't require a 386.  Current version is 2.04, available from
  ftp.simtel.net:/pub/simtelnet/msdos/graphics/2show204.zip. Shareware, $39.

Due to the remarkable variety of PC graphics hardware, any one of these
viewers might not work on your particular machine.  If you can't get *any*
of them to work, you'll need to use one of the following conversion programs
to convert JPEG to GIF, then view with your favorite GIF viewer.  (If you
have hi-color hardware, don't use GIF as the intermediate format; try to
find a hi-color BMP- or TARGA-capable viewer instead.)

The free IJG JPEG converters are available from
ftp.simtel.net:/pub/simtelnet/msdos/graphics/jpeg6b.zip (or jpeg6b32.zip
if you have a 386-or-better CPU and extended memory).  These programs will
convert JPEG to and from BMP, Targa, and PPM formats; they are DOS
compilations of the free source code described in section 15.

Handmade Software offers free JPEG<=>GIF conversion tools, GIF2JPG/JPG2GIF.
These are quite slow and are limited to conversion to and from GIF format;
thus they can't produce 24-bit color output from a JPEG.  The sole advantage
of these tools is that they will read and write HSI's proprietary JPEG
format as well as the Usenet-standard JFIF format.  Since HSI-format files
are rather widespread on BBSes, this is a useful capability.  Version 2.0
of these tools is free (prior versions were shareware), and is available
from ftp.simtel.net:/pub/simtelnet/msdos/graphics/gif2jpg2.zip.
NOTE: do not use HSI format for files to be posted on Usenet, since it is
not readable by any non-HSI software.

Handmade Software also has a shareware image conversion and manipulation
package, Image Alchemy.  This will translate JPEG files (both JFIF and HSI
formats) to and from many other image formats.  It can also display images.
A demo version of Image Alchemy version 1.10 is available from
http://www.handmadesw.com/hsi/download.html.

JPGINDEX is a useful tool for making indexes of JPEG image collections.
Available from ftp.simtel.net:/pub/simtelnet/msdos/graphics/jpgidx13.zip.