Remembering what was when over 40 episodes spread over years can be rough. Hopefully this will help you out.
Note: Astute gamers may infer spoilers from what is below. Everything is unmarked for ease of reading. You have been warned!Miscellaneous episodes
Episode 0: Freeman goes through the training course. Stops at the "Jump to Target" part as he doesn't want to be injured.
Episode 10.5: Freeman messes up a jump, falls to his death.
Before the event, episodes 1-3 (Black Mesa Inbound, Anomalous Materials)
Episode 1: Freeman rides the Tram to work
Episode 2: Freeman finds his way down to work
Episode 3: Freeman arrives at the test chamber and starts the "green apocalypse"
After the event, 4-13 (Unforeseen Consequences, Office Complex)
Episode 4: Makes his way out of test chamber.
Episode 5: Freeman continues his way out of test chamber (to the lobby) and finds a pistol
Episode 6: Jumps in a sewer, takes a lift that takes a long-ass time to go down, and gets stuck at a walkway
Episode 7: Freeman plays Extreme Hopscotch
Episode 8: Encounters bugs, electricity, maintenance shafts and a machine gun turret
Episode 9: "Okay children, class is in session! Everybody take your seats! I said everybody take your seats!! Dammit Billy, that means you too!"
Episode 10: Goes through a freezer section before finding a vending machine to get Doritos.
Episode 11: Freeman hops over lasers and finds an MP5
Episode 12: The "rescue team" arrives
Episode 13: Freeman finally gets to the surface
Freeman retreats back into the complex, 14-24 ("We've got Hostiles!", Blast Pit, Power Up)
Episode 14: Finds a shotgun, heads downward and takes a tram to a pool of "radioactive...crap"
Episode 15: Arrives at the rocket testing engine
Episode 16: Freeman wanders around silos and spots a large fan
Episode 17: Returns to the rocket propulsion test chamber
Episode 18: Restores all power to the rocket engine
Episode 19: Finally fires rocket engine
Episode 20: Jumps down a smoking hole where the alien used to be
Episode 21: Encounters the large blue alien (Gargantua)
Episode 22: Turns on the power generator for the tram
Episode 23: Freeman makes it back to track control, only to realize power is still "off"
Episode 24: Defeats the Gargantua and gets the tram working
Begins the "On a Rail" areas, 25-34 (On a Rail, Apprehension)
Episode 25: Goes around in circles on the tram, encounters a machine gun turret and ends up at a broken elevator
Episode 26: Takes the tram back to the area with the crane and heads upwards (with the tram)
Episode 27: Speaks like a pirate the entire episode
Episode 28: Loses his tram (thanks to Rocky the Rocket Ranger) and is stopped by explosives
Episode 29: Back on the surface
Episode 30: Launches satellite
Episode 31: Decides to head back down after being shot, takes another tram, and finds a big pool
Episode 32: Encounters shark-thing (Ichthyosaur)
Episode 33: Bypasses the giant pistons and discovers he's being tracked