I like Hexen, though I play on the easiest difficulty so I can concentrate on solving puzzles and avoid getting killed by monsters. :redface:
Half Life 1 is very atmospheric and immersive... The expansion packs are OK, although a bit short. What liked most about was that they offered a different perspective on the same story... (ooops, shouldn´t talk too much about HL1 cause Lee will complain
: )
I might be wrong but I think Chex Quest has been released as freeware.
System Shock 2 is a great game. Scary the first few times, on subsequent playthroughs the gameplay got me addicted. Sadly I´ve just played it to many times...
System Shock 1 is also great. Not as scary as part 2, yet also less linear. Despite quite ugly graphics (and in some levels horrible music) is quite an awesome game once you get used to the controls. It has lots of cool things that the sequel lacks (cyber hardware, cyberspace programs, jetpack, etc).