Shame the game is pretty much dead, I would pay good money for a game like it.
Either way, you can download it through steam http://store.steampowered.com/app/17..._7_suggest__13
HL2 marked the point when the expected quality of art assets went through the roof, and a lot of mod teams couldn't really match it. It's easier to work on MP mods because there's no requirement for digital acting or anything like that, so a lot of them are multiplayer mods. Mapping and modding declined with the HL2/D3 era unfortunately. Also the pipeline was (still is) atrocious.
That said Missing Information is great for a look at what HL2 might have been but last I saw the public release hadn't been updated for ages. Minerva as suggested by gundato definitely wasn't bad either. Some of the MP mods were good but I don't know if they still get regular playtime compared to the juggernauts like TF2. A lot of the more successful ones from the HL era went on to have commercial releases (like NS2).
If you like singleplayer mods I would recommend Mission Improbable. It's short but very well made and if I remember correctly it was created by 2 employees from DICE in their own time. High quality and with many custom textures and objects. Feels like an outtake from the original campaign.
There was a short single-player mod with a prison break theme, but I don't remember its name. It had a λ in it. I quite enjoyed it, though.
Also, we shouldn't forget the Stanley Parable.
never thought this game has so many excellent mods available. i can't imagine now how i managed to stay for 9 hours just by playing this game before. crazy.
Research and Development - unique. Not a single gun in sight. Entirely puzzle and antlion based. Uses the physics system of the source engine beyond anything Valve offer in the main series.
Minerva - best map design I have seen yet. Valve gave the mod designer a job, that's how good it is. And undoubtedly he had a hand in several of Portal 2's brilliant levels.
If someone could combine Minerva and R&D i'd be in gaming heaven.
Thanks everyone for the great suggestions. looks like i have plenty of play around with from this game.
NEOTOKYO patch just came out so servers should be busy for a little bit if you ever wanted to know what it's like.
Oh yes. That game is awesome.
looks like I'm late for the party. Dunno if it's one of the best but Firearms Source
open beta coming soon for Firearms:Source 2.0 so I would hold out for that as I don't think many (any?) are still playing since 1.0 was released.
Can Neotokyo or any of these mods be played without having HL2? For example if you have L2D 1 or 2 or one of the SDKs?
For Episode 2, but Nightmare House 2 was worth buying HLE2 alone. I'm going to make it a habit to play it every halloween.
Dystopia I'll echo. Played a fair bit of it back in the day. NeoTokyo might have had the style (Dys does have its own style), but it had nothing compared to Dystopia's substance. Tight team and objective based gameplay, 3 classes, 4 (main) weapons for each, oodles of implants and all the customisability that comes with that, loads of maps, meaningful Cyberspace play, polished, great UI and interface in general... from things like the hitbeeps, the map/radar, how the implants worked. Brilliant game, and a real shame they didn't get snapped up by Valve for a commercial project.
Although, the Dys Dev Team became Puny Human Games, who are currently working on Blade Symphony (JK2 inspired sword fighting/duelling game), which was one of the first vidya game Kickstarters, about 12 months before the whole Kickstarter craze took off. They succeeded and picked up a Source licence, I think. I also heard whispers that they've (re)started work on Dystopia again.
Hidden Source was also pretty good fun. The RPS Steam Chat peoples have been known to get together for a game of it, occasionally.
Not sure about other mods mentioned here off the top of my head. Generally speaking, you can install a Source mod, launch it and Steam will attempt to download additional files if needed (so if the mod requires props, textures or models from HL2:EP2 it will download those so long as you own it).
No more room in hell is fairly good, assuming that you can get it to work. Desura has a pretty good collection of HL2 mods in general, as does Steam. I agree that The Hidden is awesome. If you`re not sure about it you can watch Kilplix`s series on it. Sure, he`s a boring YT screamer most of the time but at least the videos demonstrate how the game works.