Playing with the microwave
In the kitchen near the locker room in the beginning of the game, keep using the microwave until it explodes. A nearby scientist will say "My God, what are you doing?!"
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.
You'll find some curiosities in Gordon's locker. The two books, titled "The 37th Mandala" and "The Orchid Eater" are existing novels. The author, Marc Laidlaw, is also the writer of Half-Life.
The baby on the picture is Isabel, daughter of designer Harry E. Teasley.
Gabe Newell texture
After you take the elevater with headcrabs jumping from above, put the noclip cheat on, as well as the r_fullbright 1 cheat (to more easily find the isolated dark room). Go through the wall by the doorway and the metal barrels, look for pink cubic room below and enter it. All of the walls, ceiling, and floor is filled with Gabe Newell texture, co-founder of Valve. The room is actually used to spawn creatures before the player sees them.
By the door of the flooded kitchen in the office complex you'll find a board "Dr. Newell Chaos Theory R&D". This is a reference to Gabe Newell, co-founder of Valve.
In many places throughout the game, you can find walls which have Valve (the developer) vaguely displayed on them. The image below, for example, was taken in the ceiling space in the office complex. The name also appears in other places, such as shown on the second image, which is by the entrance of the freezer.
Riller was here
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.