The EMP grenade in Team Fortress Mega. Basically, when the grenade detonated it was harmless but had the effect of setting off any explosive ammunition in its radius. This explosion did normal damage but also, itself, set off additional explosive ammunition. This included ammunition lying in crates on the ground, in backpacks dropped by enemies or whatever ammunition some hapless bastard happened to be carrying at the moment. In a game where blast radius was generally measured in a meter or two, the EMP had the potential to carry a blast from one end of any map to the other. The damage done by an explosion was proportional to the amount of ammunition found but this was largely irrelevant. A soldier carried enough explosive ammunition to kill a player of any class a few dozen times over.
A rarely seen iteration of the TF Mega concept was TF custom which let players build a custom class using points. Basically they would select legs (for base speed), weaponry, ammunition stocrres, grenade types, armor type (back then different classes wore different armor. Pyro's wore asbestos which made them generally immune to fire while demolitions men wore blast armor making them highly resistant to explosives. There was also kevlar (generic) and wooden (the bullet firing weapons in the game actually fired nails back then)), and so forth. One could build a class with only the most primitive weapons for defense but absolutely loaded with explosives (grenades, rockets, hand grenades and shotgun shells), a fast set of legs and enough health and armor to ensure you'd arrive in one piece. Then you could simply run into a strong point clutching an EMP grenade and nuke an entire base. I've seen diabolical defense grids set up in the ramp room of 2fort crumple in a single run, grids that would normally take the concerted effort of an entire team of skilled players a dozen lives each to breach. It was basically like giving the player their own thermonuclear device.