I've started work on my first Half Life mod, currently titled "Miasma", its a single player and multiplayer modification based around the deliberately typical "zombie survival scenario".
Combat will be highly involved, involving hitting strategic targets on your enemy based on what weapon you are using. All new weapons, too.
Multiplayer will involve players teaming up against the NPC zombies, each player starting away from others in areas such as a town. Players must find equipment, weapons, npcs and other players, and move towards some goal in the map, such as a rescue helicopter.
At the moment it's myself doing modelling, mapping, programming and just about everything else, and a friend of mine working on sound and music.
Here's some shots, just because shots are fun: