Half-Life 2: Sandbox is an open-source sandbox modification of the Source Engine which provides Lua as a scripting platform for developers to quickly prototype game ideas. The minimalistic game includes a sandbox gamemode for playing with the many aspects of the Source Engine.
Adnan's Rotational Gravity Gun
Adnan Zafar
adnan.f.zafar AT gmail DOT com
https://web.archive.org/web/20150222194420/http://skew.telefragged.com/mods/hl2/ARGG//
ARGG is used by Half-Life 2: Sandbox to implement rotational functionality for the physics manipulator.
Open Steamworks
Open Steamworks is used in Half-Life 2: Sandbox for mounting additional game content through Steam's abstraction layer rather than the game engine's filesystem class, which prevents non-fatal engine errors.