In Half-Life 2: Episode 2, Dr Magnusson accuses Gordon Freeman of a "debacle at Black Mesa" involving "a certain microwave casserole." This refers to a moment in the game's opening in which the player is able to tamper with a microwave, causing the food inside it to explode.
Contributed by Retsof-Noraa
The Half-Life series draws some inspiration from various adaptations of H. G. Wells's The War of the Worlds; the Vortigaunts who appear throughout the series are almost identical to the Mor-Tax aliens from the War of the Worlds TV series.
Contributed by RayMonkay