The Half-Life franchise, created by Valve Corporation, comprises a series of first-person shooter (FPS) games along with expansion packs and spin-offs. Here's a list of the main entries and additional titles in the franchise up to the knowledge cutoff in early 2023:

Main Series:

1. Half-Life (1998)
2. Half-Life 2 (2004)

Expansion Packs for Half-Life:

1. Half-Life: Opposing Force (1999) - Gearbox Software developed this expansion, where players control Adrian Shephard, a U.S. Marine.
2. Half-Life: Blue Shift (2001) - Another Gearbox-developed expansion, focusing on the character Barney Calhoun, a Black Mesa security guard.
3. Half-Life: Decay (2001) - An expansion by Gearbox designed for co-op gameplay, available initially only on PlayStation 2. It was later ported to PC by fans.

Half-Life 2 Episodes (Continuations of Half-Life 2 story):

1. Half-Life 2: Episode One (2006)
2. Half-Life 2: Episode Two (2007)

Spin-Off Games:

1. Half-Life Alyx (2020) - A VR spinoff that acts as a prequel to Half-Life 2.
2. Portal (2007) - While initially not marketed directly as a Half-Life game, it is part of the same universe and includes references that tie it to the events and lore of Half-Life. The game introduced players to the Aperture Science laboratories and the now-iconic portal gun.
3. Portal 2 (2011) - The sequel to Portal that further expanded the lore connecting the Portal series to the Half-Life universe, providing more information about the shared world’s history and the relationship between Aperture Science and Black Mesa.

While the Portal games are not direct Half-Life gameplay experiences, their shared universe and storyline details grant them a place as spin-offs or sister games within the broader franchise.

Unofficial Fan-Made Games:

1. Black Mesa (2015) - Initially developed as a fan project, this game received Valve's endorsement. Black Mesa is a recreation of the original Half-Life game using the more advanced Source engine (the same engine used for Half-Life 2).