Garry's Mod

Garry's Mod requires the user to own one Source engine based game (games including Source SDK), such as Half-Life 2, Portal, or Team Fortress 2, and that the Source game has been used at least once. This ensures that the necessary program files are provided to Garry's Mod. The game can be bought in a pack bundled with other Source games or purchased over Steam.

There are several gamemodes for Garry's Mod providing a wide variety of gameplay. There are dedicated servers for:

• Roleplaying
• Construction
• Build Traps
• Deathrun
• Racing
• Sandbox
• Zombie Survival

Garry's Mod features multiplayer and single-player. Alongside the Half-Life 2 games, it is possible to use assets from other Source Engine-based games such as Left 4 Dead, Team Fortress 2, Day of Defeat: Source, Portal, Zeno Clash, and Counter-Strike: Source.

Modding

Garry's Mod has a vibrant mod commnity. Mods (also referred to as adddons) can range anywhere from new weapon and character models to entirely new gamemodes and tools for content creation.

The majority of addons can be obtained from  http://www.garrysmod.org/downloads/ and http://www.facepunch.com. The constant flow of uploaded user created material makes the game very customisable.