From Valve Developer Community
Half-Life: Uplink is a chapter originally cut from Half-Life. The Uplink chapter is a series of three maps which were modified and released as a standalone demo for Half-Life. Half-Life: Uplink was later unofficially released in an "Uplink Lite" version that could be run as a Half-Life mod.
The demo takes place in areas of Black Mesa and allows the player to use a number of the weapons and fight a number of the enemies from the final game release.
Half-Life Decay has a chapter similar to Half-Life: Uplink.