Source Ports for Doom are almost a gaming essential to play any of the new maps being made. Since the Source code for the game was released in 1997, there have been many of these improved Doom programs written. Each has their own good and bad points, and every Doomer out there seems to have his or her favorite.
Here is a list of the most popular Doom Source Ports, along with where to download them from and a little information about each.
JDoom: http://www.doomsdayhq.com/index.php
This was the first source-port I ever found. (It used to be required to play the Doom64TC.) JDoom has some of the best graphics and lighting effects you will ever see in a Doom game. It also has add-ons for 3D models, rocket trails, fireball effects, and (my favorite,) More Blood!
Legacy: http://legacy.newdoom.com/legacy.php
Legacy looks more like the vanilla Doom2 we all know and love. The graphics are not enhanced much from the original game, but it does have lots of special features like real 3D floors over floors, lighting effects, and deep water. This port is extremely stable and will handle the largest and most complicated maps without slowing down at all.
Legacy also has a limited OpenGL mode, but I can't say much in favor for it. When it does work it looks terrible most of the time so I never use it except in software mode.
Risen3D: http://r3d.doomwadstation.com/
Risen3D is based on JDoom and looks pretty much the same. The difference is mainly in how fast it runs and how stable it is compared to JDoom. This also supports more true 3D effects than JDoom does.
ZDoomGL: http://zdoomgl.mancubus.net/
ZDoomGL does almost everything that ZDoom does, only the graphics are displayed in OpenGL. The GL version is programmed by entirely different people than those who write ZDoom, and ZDoomGL is still several versions behind, so not all features have been implemented yet.
While this doesn't look quite as nice as JDoom or Risen3D, it's faster, more stable, and will play the most advanced and complicated maps being made with special effects and features that are almost unbelievable.
GZDoom: http://grafzahl.drdteam.org
This is the newest and most advanced ZDoom port available with lots of options. It looks almost identical to ZDoomGL, but runs faster, cleaner, and is based on the most recent version of ZDoom. Very highly recommended.
ZDoom: http://www.zdoom.org/about.html
ZDoom looks like crap, pure and simple. It's blurry and washed-out looking. If you want a classic Doom2 look when playing, use Legacy.
There are many many special features of ZDoom however, which make it a favorite among the best map designers. It not only supports true 3D floors, but swimmable water, flying, angled sectors, earthquakes, and Hexen style scripting.
As far as I am concerned, the main purpose of even downloading ZDoom is to provide the basic files needed to run ZDoomGL.
Simple ZDoom Launcher: http://members.lycos.co.uk/Enjay001/
"SZL" is not a source-port at all, but a launcher for ZDoom, GZDoom, and ZDoomGL. All the other source-ports listed above come with a launcher program to choose the wads and options you want to play. With ZDoom, one usually has to just drag and drop the files onto the .exe to start the game. This program gives you an actual launcher instead.
(The download link is about 3rd from the bottom on this page by Enjay. It wouldn't have been cool for me to just link to the file.)