I was just looking over some HL2 mods for the first time. I haven't modded a game since maybe HL1 or quake 2.

This one looked pretty interesting for multiplayer HL2
http://www.moddb.com/mods/obsidian-conflict/downloads/obsidian-conflict-135-full-installer


and I also found this today
http://www.blackmesasource.com/

Half-Life was a great game. They don't make them like they used to. I miss the games of the 90's.

Garrys mod is awesome -- shame they made it a pay addon...

Nod, never even got into hl2 that much. tf2 i was semi hyped for, but its too simplistic now.
I am somewhat following natural selection 2. that was my fav hl1 addon.
for those that dont know, picture and rts starcraft type game but fps.
complete with building stuff over gas nodes and collecting resources, researching upgrades, etc

Alien swarm is the latest free mod that's all the rage, for multiplayer.

There have been plenty of others:

Golden eye (source)
http://www.moddb.com/mods/goldeneye-source

These are very good single player mods that I've played beginning to end:

Minerva
http://www.hylobatidae.org/minerva/filegoatmatic.php?filename=metastasis_patched.exe
required this addon:
http://steampowered.com/v/index.php?area=game&AppId=380


Riot Act
http://uoz.multiplayer.it/ra/index.php?content=download


Rock 24
http://www.fileplanet.com/170593/170000/fileinfo/Half-Life-2---Rock-24-mod



But by far one of the best ones I've played in Research and Development.

Awesome HL2 puzzle mod created by a game developer (Matt Bortolino) that once was part of the team that made Hellgate London.


Download the installer for windows here:

http://www.filefront.com/14083899/moddb/Half-Life-2-Research-and-Development-Mod/

Also Alien Swarm you can download for free from the steam store.