All patches for the retail versions of Doom II: Hell on Earth (as well as Doom and Final Doom) can be found here.
When using the keyboard and mouse configuration in Doom or similar games on DOS or through DOSBox, vertical mouse movement causes the player to move forward and back, which can make mouse aiming difficult. novert is a DOS TSR that disables vertical mouse movement to prevent unnecessary player movement.
MOUNT C "."
DOOM.EXE -config MOUSE.CFG
If using a source port, it is recommended to use any of these following launchest to easily load the WADs or PK3 files, change their loading order, add the command lines or even set up the starting level. Some of these even allows you to save the list of mods you have added.
The following wads are designed to fix some sort of game data bug or a feature short coming of the game. They do not add any levels/change play style and thus are generally compatible with most user add-ons.
If using the ZDoom source port or its derivatives, The Master Levels Menu Interface is an in-game menu that serves to replace the old DOS program that would launch a chosen WAD of the Master Level for Doom II expansion, this mod allows you to play them in one sitting without restarting ZDoom or similar source ports and loading each individual WAD file.
If using the GZDoom or QZDoom source ports, you can use JP LeBreton's WadSmoosh tool to combine any of the official DOS-era Doom games' IWADs (Doom, Doom II: Hell on Earth, Master Levels for Doom II, and Final Doom) that you own, as well as the No Rest for the Living WAD from Doom 3: BFG Edition, into a single PK3 IWAD with each game selectable as episodes. The bonus secret levels from the Xbox port of Doom 3 and Resurrection of Evil expansion can also be integrated. The tool along with a guide to use it can be found here.
A freely redistributable replacement of Doom-like assets is available at Freedoom, forming a completely free and open source game. Art assets from the original game such as the levels, sounds, textures, etc., remain under copyright. They can be purchased from Steam, GOG.com, or copied from an original game disc.
Depending on your choice of engine, you can usually change the field of view with a console command, or even in the options. Try fov <1-179> or gr_fov <1-179>.
ZDoom seeks to support multiple monitors out of the box since version 2.5.0. Widescreen Gaming Forum discusses multi-monitor set ups with GZDoom.
The following lists source ports that supports borderless fullscreen windowed, either natively or though third-party programs.
Run the program as administrator