Also, you can donate to the site now. How nice for you!

This scrap with the Antlion Guard (or Myrmidont, if you're snooty) is about the closest thing to a traditional boss-fight you'll find in Half-Life 2. You've jumped or waded through a level of nothing but antlions, you reach this large gulch, and then this ur-antlion pops out and it's time for a showdown. While you scramble around trying to kill it, it summons lesser antlions periodically. When you beat it, the door at the far end opens. Boss Fight, pure and simple.

There are other bossish fights in the game, I guess. Gunships and Striders are sort of boss-esque, and a lot of times you can't continue until you beat them, but this Antlion Guard battle really feels like a classic, Megaman-type boss fight, where you take on a level of similar underlings in an environment tailored for them, you reach the room where the boss has been specifically waiting for you, you're stuck in it until you kill him, and when you win, you get his power, sort of.