Dear Esther - An experimental "ghost story" created as a research project at the University of Portsmouth initially released as a free modification in 2008, a longer commercial version was developed and released in 2012.
Coastline to Atmosphere - A mod set in the Half-Life 2 universe, following Gordon Freeman after the events of Half-Life 2.
Black Mesa - A recreation of Half-Life using the more advanced Source engine.
Instead of a portal gun, the player solves puzzles using a mix of these gels that they can spray onto surfaces with a tool they carry.
Aperture Tag - A modification based on Portal 2 that recreates the essence of the game Tag: The Power of Paint, the inspiration for the various gels used in Portal 2.