First one - DooM 2D, was made in 1996 in Russia for MS-DOS by some students just for fun.
Years later another guys remade it for windows and named it DooM 2D: Forever.
There was no multiplayer, and so 2 another guys from D2D:F forum made up a multiplayer doom 2d, for DM like modes. DooM 2D Multiplayer.
Why im posting it here? Maybe some of you will want to try these games

=1. DooM 2D=
Sadly, i cant find english version right now, and i suggest you to try instead second one i mentioned, with hotseat and campaigns:
=2. DooM 2D: Forever=
Some features:
Screenshots (sorry for the bad quality):
Videos:
Deathmatch with bots
Map 01 (Video from old version where was no campaigns)

English forum part
=3. DooM 2D Multiplayer=
D2DMP is a simple online 2d platform shooter. As you've probably guessed, it is inspired by ID Software's Doom series, and in fact is a remake of an old russian Doom II demake called 'Doom 2D', which had almost nothing to do with the game it was named after, aside from some assets. It was made by a bunch of russian coders (called Prikol Software) in 1996, and was a nice platform-puzzle-shoot-em-up-thingy.
Authors completely remade it using 39DLL and GM8, and now you can crush, kill and destroy up to 24 of your friends over the net like in the good ol' times.
It is also open-source and is being updated constantly, so you can watch me gradually failing at online programming, or just use the networking part as an example for your future games.
Features:
Screenies:
Video:
http://www.youtube.com/watch?v=kA2YEVRCX3c&feature=player_embedded

0.6 125 (About 8.9 MB)
It contains the game, the map editor, the documentation and the .gmk sources.
Also, check these out:
All currently present user-made maps in one mappack (~5.40 MB, last updated 11 Jun 2012).
All currently present user-made player skins in one skinpack (~200 KB).