Overview
This guide contains almost if not all HL2 Enititys, all from the Valve Developer Community.
Everything
ai_ally_manager
ai_battle_line
ai_changehintgroup
ai_changetarget
ai_citizen_response_system
ai_goal_actbusy
ai_goal_actbusy_queue
ai_goal_assault
ai_goal_follow
ai_goal_injured_follow
ai_goal_lead
ai_goal_lead_weapon
ai_goal_operator
ai_goal_police
ai_goal_standoff
ai_npc_eventresponsesystem
ai_relationship
ai_script_conditions
ai_sound
aiscripted_schedule
assault_assaultpoint
assault_rallypoint
combine_mine
cycler_actor
env_alyxemp
env_ar2explosion
env_citadel_energy_core
env_entity_dissolver
env_flare
env_global
env_gunfire
env_headcrabcanister
env_muzzleflash
env_rotorwash
env_speaker
env_sporeexplosion
env_starfield
env_terrainmorph
filter_combineball_type
func_bulletshield
func_combine_ball_spawner
func_healthcharger
func_lookdoor
func_monitor
func_recharge
func_tank
func_tank_combine_cannon
func_tankairboatgun
func_tankapcrocket
func_tanklaser
func_tankmortar
func_tankphyscannister
func_tankpulselaser
func_tankrocket
func_vehicleclip
generic_actor
grenade_helicopter
info_apc_missile_hint
info_darknessmode_lightsource
info_radar_target
info_snipertarget
info_target_gunshipcrash
info_target_helicopter_crash
info_target_vehicle_transition
info_teleporter_countdown
item_ammo_357
item_ammo_357_large
item_ammo_ar2
item_ammo_ar2_altfire
item_ammo_ar2_large
item_ammo_crate
item_ammo_crossbow
item_ammo_pistol
item_ammo_pistol_large
item_ammo_smg1
item_ammo_smg1_grenade
item_ammo_smg1_large
item_battery
item_box_buckshot
item_dynamic_resupply
item_healthcharger
item_healthkit
item_healthvial
item_item_crate
item_rpg_round
item_suit
item_suitcharger
logic_achievement
logic_choreographed_scene
logic_scene_list_manager
monster_generic
npc_advisor
npc_alyx
npc_antlion
npc_antlion_grub
npc_antlion_template_maker
npc_antlionguard
npc_apcdriver
npc_barnacle
npc_barney
npc_blob
npc_breen
npc_bullseye
npc_citizen
npc_clawscanner
npc_combine_camera
npc_combine_cannon
npc_combine_s
npc_combinedropship
npc_combinegunship
npc_crabsynth
npc_cranedriver
npc_crow
npc_cscanner
npc_dog
npc_eli
npc_enemyfinder
npc_enemyfinder_combinecannon
npc_fastzombie
npc_fastzombie_torso
npc_fisherman
npc_gman
npc_grenade_frag
npc_grenade_magna (Former name for weapon_striderbuster)
npc_headcrab
npc_headcrab_black
npc_headcrab_fast
npc_heli_avoidbox
npc_heli_avoidsphere
npc_heli_nobomb
npc_helicopter
npc_hunter
npc_hunter_maker
npc_ichthyosaur
npc_kleiner
npc_launcher
npc_magnusson
npc_maker
npc_manhack
npc_metropolice
npc_missiledefense
npc_monk
npc_mortarsynth
npc_mossman
npc_pigeon
npc_poisonzombie
npc_rollermine
npc_seagull
npc_sniper
npc_spotlight
npc_stalker
npc_strider
npc_turret_ceiling
npc_turret_floor
npc_turret_ground
npc_vehicledriver
npc_vortigaunt
npc_zombie
npc_zombie_torso
npc_zombine
path_corner
path_corner_crash
player_loadsaved
player_speedmod
player_weaponstrip
point_antlion_repellant
point_apc_controller
point_bugbait
point_camera
point_combine_ball_launcher
point_flesh_effect_target
point_push
prop_combine_ball
prop_coreball
prop_scalable
prop_thumper
prop_vehicle
prop_vehicle_airboat
prop_vehicle_apc
prop_vehicle_cannon
prop_vehicle_choreo_generic
prop_vehicle_crane
prop_vehicle_driveable
prop_vehicle_jeep
prop_vehicle_prisoner_pod
script_intro
script_tauremoval
scripted_sentence
scripted_sequence
scripted_target
test_sidelist
trigger_physics_trap
trigger_rpgfire
trigger_vphysics_motion
trigger_waterydeath
trigger_weapon_dissolve
trigger_weapon_strip
vehicle_viewcontroller
weapon_357
weapon_alyxgun
weapon_annabelle
weapon_ar2
weapon_brickbat
weapon_bugbait
weapon_crossbow
weapon_crowbar
weapon_frag
weapon_physcannon
weapon_pistol
weapon_rpg
weapon_shotgun
weapon_smg1
weapon_striderbuster
weapon_stunstick
All credits to [link]. Please note some of these are not in Half-Life 2, but its episodes.