This is a list of all known game modes and modifications for Half-Life 2: Deathmatch.

This likely isn't a complete list, but this should help you find a server that you'll like.

Most of these game modes have crossover with other Source Engine multiplayer games, particularly Counter Strike: Source.

At some point [likely pre-2010], the vast majority of the less competitive game modes like Death Race moved to Garry's Mod; where they presumably retained some level of popularity.

In [20XX], Garry's Mod got an update that made the server browser prefer more popular modes (such as Trouble in Terrorist Town and DarkRP) by default, causing some of those mods to fall into obscurity.

As of 2022, there are still Half-Life 2: Deathmatch servers being hosted and played on, usually customised to some degree.

The base game of Half-Life 2: Deathmatch only includes two modes by default; Deathmatch, and Team Deathmatch.

The only way to change between these base game modes is with a single console command (mp_teamplay).

To get around this limitation, there are two methods that are used to create new modes, usually with a combination of both methods.

These are Source Mod plugins that can change a large variety of game properties that usually aren't accessible.

They can be obtained from the SourceMod Plugin Page[www.sourcemod.net]

These are pre-made map files that contain logic for running a custom game mode, such as Jora_'s HL2:DM CTF Prefab[gamebanana.com].

This is the layout of mode descriptions.

Abbreviation: If the community uses an acronym to refer to this mode, it's mentioned here.
Map Prefixes: Common map naming scheme. Technically not required, or even consistent.

Mode description without formatting tools.

Square brackets - My notes
Question Marks - Missing info

Abbreviation: DM
Map Prefixes: dm_*, aim_*, pg_*

Free for all deathmatch. Kill to score points, suicide to lose them.

May have a score limit, time limit, or none at all.

This is a vanilla game mode (mp_teamplay "0")

Abbreviation: TDM
Map Prefixes: dm_*, aim_*, pg_*

Standard deathmatch rules but with teams instead; Combine (Blue) versus Rebel scum (Red).

This is a vanilla game mode (mp_teamplay "1")

Abbreviation: HK
Map Prefixes: dm_*, aim_*, pg_*

Also called high kill, this a deathmatch variant that generally increases the lethality of all weapons.

May also change other game mechanics like weapon properties, player movement, or adding low gravity, depending on the server.

Abbreviation: CTF
Map Prefixes: jctf_*, real2ctf_*

Two teams try to both defend, and steal each other's flag for points.

Abbreviation: N/A [AA?]
Map Prefixes: dm_*, aim_*, pg_*

A Deathmatch variant that changes the scoring to be more biased towards skillful play.

The winner is determined by a combination of factors, including - average movement speed, headshots, crowbar kills, kills per minute (KPM) and Kill/Death Ratio (KDR).

Additionally, there are gameplay changes. Weapon spawns are altered (the shotgun now spawns where a RPG would for example), and grenades are now earnt through headshot kills.

Abbreviation: N/A [AM?]
Map Prefixes: 0adm_*, 1adm_*

A Quake-like modification for standard Deathmatch that adds player bright skins, Quake-like powerups, pickup announcements and notifications.

The mode is likely depreciated, as most of its features have been folded into the SoCal mapper's prefabs (referred internally as "Prefabulous").

Abbreviation: N/A [DR?]
Map Prefixes: deathrun_*

An asymmetrical team mode, where one team runs an obstacle course and the defenders pushes buttons to activate traps, with the goal of reaching the end of the map.

Some maps may end with a fight between the teams.

Abbreviation: N/A [CP?]
Map Prefixes: ???

A King of the Hill and Deathmatch hybrid.

Abbreviation: ???
Map Prefixes: ???

Like actual football; two teams kick a ball (roller mine) around with their Gravity Guns into the other teams' goal.

Abbreviation: Bots
Map Prefixes: dm_*, aim_*, pg_*

Simulated player-versus-player matches against AI players.

Abbreviation: coop
Map Prefixes: coop_*

(This is the standard co-op mode.)

Players fight standard Half-Life 2 campaign enemies in stages, with the goal of getting to the end of the map.

Enemies typically spawn in waves, with all them being needed to be killed to get to the next stage.

AI may have their properties adjusted such as health and damage to compensate for high player counts.

Abbreviation: N/A
Map Prefixes: ???

Similar to co-op maps based on Half-Life 2, zombies mode is played on larger maps that encourage players to build defenses out of props.

Non-standard game modes.

Abbreviation: N/A
Map Prefixes: z_*

Attach watermelons to your fishing rod, and sink it into the water. See what you can fish out.

This mode is mostly about player interactions with whatever they get.

Abbreviation: N/A
Map Prefixes: jm_*

Precision platforming across multiple stages, with each stage usually having a small time limit between obstacles before you reset to the last checkpoint.

Abbreviation: RP
Map Prefixes: rp_*

Like playing pretend in a fictional world, featuring their own systems like laws, a functioning economy, and so forth.

Abbreviation: N/A
Map Prefixes: puzzle_*

Various challenges you need to figure out, depending on the map.

Abbreviation:
Map Prefixes: surf_*

Ride slanted surfaces to gain speed. Use this speed to jump across gaps!

Comes in racing and combat surf variants.

These are gameplay changes that don't directly change the mode's objective, such as low gravity.

A server plugin that adds multiple custom weapons to whatever game mode you're in.

This is mostly used on modern co-op servers. Weapons can include medkits to heal teammates with a sort-of beam, the tau cannon, the HL2 beta rifle, a cannon, fists, cat-butt gun, and so on and so forth.

Allows players to use cheats, usually to spawn entities. Configuration scripts (.cfg) were commonly used to quickly build structures such as towers.

There isn't much info about this mode; presumably, the player base moved to Garry's Mod Sandbox servers.

Crossbow modification that rapidly increases its fire rate, among other changes.

Common additional crossbow changes include high damage buffs (this makes props fly across the map when hit), trails following the bolt projectile, explosions on impact, added bolt penetration, and the bolts adding additional burning damage over time.

Adds the ability to fire a grappling hook at walls, to either swing with or stay stuck high onto a wall.

This is sometimes coupled with a parachute plugin.

Allows the Gravity gun to be used to both push other players, and rocket jump with.

Unlimited bullets; sometimes combined with a bottomless clip.

Reduces the gravity of all players and objects in the game, usually by half.

Gives the 357 Revolver non-lethal explosive shots that can be used to jump with, similar to Ratz Instagib.

Adds the ability to launch yourself at high speeds by running props into a wall, replicating a pre-Orange Box era engine exploit.

These are other notable server modifications that interact with players.

Match system plugins allows quick setup of both casual and competitive game modes for players.

Common plugins for this are XMS[github.com] and PMS (couldn't find a link to PMS repository, but it's included in hl2dm.net's Public Match Server Pack[hl2dm.net]).

Servers that don't connect to VAC, but allow people who own the game on Steam to join.

Naturally, they're infested with cheaters.

Resources I used to make this guide.

Valve Developer Commentary page for known map prefixes. The Counter-Strike: Source section is the most relevant for Half-Life 2: Deathmatch.

Link: https://developer.valvesoftware.com/wiki/Map_prefixes

The maps section for Source Engine games has subcategories for popular game modes.

HL2:DM Section: https://gamebanana.com/mods/cats/5327
CS:S Section: https://gamebanana.com/mods/cats/5535
Garry's Mod Section: https://gamebanana.com/mods/cats/5305

These are notable map naming schemes.

These are maps that share a similar theme.

killbox - These are usually rectangular boxes with lots of props and weapons scattered everywhere. The layout usually has minimal cover, and typically with an increased emphasis on height.
no-prop - Maps that lack interactive props that can be influenced by the Gravity Gun. (Note: Maps can be made into no-prop variants with the use of server plugins, such as Stripper Source)

Explanations for these game modes can be found earlier in the the guide.

0adm_*: Map uses prefabs associated with the SoCal #1 server.
1adm_*: Map uses prefabs associated with the SoCal #1 server.
aim_*: Unsure. The prefix suggests the mode is based on a Counter Strike custom game type, but most HL2:DM maps with it's name are standard maps (e.g. aim_arena_reloaded, aim_arena_new). Possibly refers to the layout being arena-like?
coop_*: Co-op, versus AI
deathrun_*: Deathrun
dm_*: Standard Half-Life 2: Deathmatch map
jctf_*: Capture the Flag
jm_*: Jump
rp_*: Role play
pg_*: Map is ported from Adrenaline Gamer 2
puzzle_*: Puzzle map
real2ctf_*: Capture the Flag
z_*: Fishing

_a* - Alpha Release
_b* - Beta Release
_rc* - Release Candidate
_prefinal - Similar to Release Candidate
_final - Author's final release of the map

CLANG-CLANG - Writer
Xeogin - HL2:DM Historian