If you download a mod and it doesn't work, make sure you didn't miss a patch. A lot of these mods were broken by engine updates and require patches, some of them unofficial. I'll also add new mods as they come out, so be sure to check this list every few months for new good mods.
Let me know if I forgot a good mod, and I'll add it to the list. Mods marked Highly recommended are just ones I personally enjoyed, there's nothing really objective about it. Look over all of them because there are some real gems in here. You're better off just visiting each mod's page to find out what they're about. Originally I was going to write a short description for each mod, but it didn't look good. Someone else will have to do a list for multiplayer. I'm only doing singleplayer mods because I haven't played many multiplayer ones. Opposing Force - Planet (Soundtrack) / Licensed under Creative Commons: By Attribution 4.0 License /licenses/. OBS (what I record with): / - Music: (In order of appearance). Downloads: *You need to own Half-Life and have it installed to play USS Darkstar* Mod: Half-Life: /app/70. And even though these days, mods have custom skins, textures, and a whole load of other stuff, this one still manages to remain good looking despite how old it is. Video without voice: USS Darkstar is a really old mod for half life, and pretty impressive for it's time since it was a bit of custom dialogue, clean looking maps, and overall good aesthetic. It's a very old mod of Half-Life, except it takes place in space.