Half lifeis an FPS series developed by Valve. The first game, Half-Life, was released in 2000. There has since been many sequels released. Portal and Portal 2 are considered spin-offs of the series, and these games are set in the same universe.

1998 Half-Life
1999 Half-Life: Opposing Force
2001 Half-Life: Blue Shift
2004 Half-Life 2
2006 Half-Life 2: Episode One
2007 Half-Life 2: Episode Two
TBA Half-Life 2: Episode Three
TBA Half-Life 3