Enigma is a single player mod for half life 2 that takes you through many challenges and puzzles. It's made to be a horror adventure game, and will contain both puzzels, battles and a storyline. Check the mod page to see more about it.
At the moment, i'll post a little teaser screenshot for you ;)
If you want more info, feel free to contact me at Acegikmo_Trackmania@hotmail.com or in the mod page comment section :)