Quake 2

In the hydrophobia level in the multiplayer mode.
To get the BFG 10k , in the zone that is red before the shallow pit of water, fire a automatic weapon on the left side of the red hallway. A panel will open and you get the Big Fricken Gun!!!

This is really a tip for Quake 2. Ever seen those ledges that try as you might you just can't get up? You know the Rocket Launcher fires you back a bit when you shoot well jump when you fire and you will be able to fly up to high ledges. Just for a laugh really!