The story of Half-Life continues today with the Valve-approved fan game Prospekt.
The $10 game was developed by Seacorp Technologies, consisting of just one person — Richard Seabrook. Seabrook got permission from Valve to use assets and characters from the Half-Life series.
Prospekt is a direct sequel to Half-Life: Opposing Force, an expansion to the original game told from the perspective of the game’s less-than-trustworthy military. It was developed by Borderlands developer Gearbox Software, and ended with its hero getting put on ice by the Half-Life mystery figure G-Man.
This standalone expansion/sequel picks up with that protagonist — Adrian Shephard — being pulled into the setting of Half-Life 2. According to the Steam page the story is “comparable to Half-Life 2: Episode One in length.”