Sandpit is a sandbox mod for Half-Life. Spawn and pose monsters, models and sprites from Half-Life and Opposing Force.
This review may contain spoilers
When Gary's Mod was made for Half-Life 2, It began the excitement of the "Sandbox" mod, Half-Life itself never had such a mod to talk of..until Sandpit was made in 2008.
Sandpit is simply the "Gary's Mod" of the Gold-source engine, By using a Command menu you can spawn any kind of monster from Guards and headcrabs, to Gordon Freeman and Gargs.
You start any and every game with all the Half-Life weapons give to you, Sandpit can be further enhanced by changing the wads to include Opposing Force, Gunman Chronicles, Sven-Coop, Blue-Shift, Decay, or X-Half-Life Deathmatch content.
The only problem I can find is that V2.6 is the most stable version while more modern ones like V3.4 remove several features that make Sandpit fun. (like the command menu)