About a third of the way through the "On a Rail" segment (level c2a2c), there is a place where you can go under a ramp, break a box and a grate and drop down into a vent shaft. Follow it and eventually come to an army grunt body. On the left side against the back wall is writing in the texture on the wall. Toss a few grenades at it to see it more clearly on a blackened wall - you'll find the name "Riller" embedded in the wall. This is a reference to Dave Riller, level designer of Half-Life.
In the locker room in the beginning of the game, the names on the lockers are the last names of the Half-Life design team. You'll find (Dario) Casali, (Greg) Coomer, (Ken) Birdwell, (Chuck) Jones, (Mona Lisa) Guthrie, (Douglas R.) Wood, (Steve) Bond, (T.K.) Backman, (Karen) Laur, (Dave) Riller, (Harry E.) Teasley, (Kelly) Bailey, (Brett and/or Erik) Johnson, (Jay) Stelly and (Marc) Laidlaw. The final locker, labeled Freeman, is of course the player's locker.
