Single Player

Area 51
In production Half-Life modification continuing the original storyline.
http://www.angelfire.com/ns/tsblood/

Borked Hazard
A single player mod where you play as a member of staff in a training facility for Bio-tec research.
http://hosted.planethalflife.gamespy.com/borkedhazard/

Poke646
A Half-Life single player modification with new weapons and other details.
http://www.poke646.com/

Project: Quantum Leap
Multi-authored, single-player Half-Life episode flouting any concept of continuity.
http://www.unquenque.com/quantum/