Never said anything about not allowing demos so:
Serious Sam: The First Encounter demo
Serious Sam: The Second Encounter demo
Call of Duty 4 demo
Operation Flashpoint: Cold War CrCIA demo
Quake II demo
- Operation Flashpoint has multiplayer and singleplayer, but multiplayer is LAN-only as far as I know.
- Serious Sam has a button for multiplayer on the menu, but I'm not sure if it's LAN-only or if it can be played online.
Source (Half-Life 2) mods:
Research and Development
SMOD
Zombie Master 1.2.1
Synergy 1.1
Zombie Panic!: Source
Free, online games
America's Army
America's Army 3
Half-Life 2: Deathmatch
- HL2DM is only free by a certain download link and only if you have an Nvidia or ATI graphics card.
- NVIDIA link:
http://steampowered.com/nvidia/ - ATI link:
http://steampowered.com/ati_offer1a/- HL2DM and AA3 * require
STEAM.- The listed HL2 mods will work as long as you have the Source SDK Base installed on Steam. The SDK is included with all Source games (HL2DM is a Source game, but the free version doesn't include the SDK, because, well, it's free.)
* Correct me if I'm wrong.