The gelatinous cube, with relation to Dungeons and Dragons, is one of the creepier monsters featured in the whole game. Sure, dragons can toast you, evil wizards can disintegrate or wish you to oblivion, and hordes of angry trolls can tear your hapless adventurer limb from limb, but only the vile, emotionless mass that is the Gelatinous Cube can absorb you at an excruciatingly slow pace.
With its surface coated with an effective paralytic, the cube need do nothing but steadily roll down hallways until it traps something. If your player character finds himself trapped, the best thing to do is burn the cube. If you can't do this, you can try to cut your way through, but once you're paralyzed, it's over, and you're jelly.
The Monstrous Manual (and in the old days, the Fiend Folio) always gave great descriptions of this creature, mentioning nightmarish "legendary" gelatinous cubes that may grow to several hundred cubic feet of volume. The only thing that bothered me more was the idea of green slime converting victims to slime within a matter of a minute or so.