NIKITA remote-controlled missiles in Metal Gear Solid and MGS2.

These gadgets are a hell of a lot of fun. They're an absolute blast, especially since they can be flown around in first-person view, but they're exceptionally impractical as actual weapons. The projectile is very slow, and there's no pitch control, so avoiding the terrain can be a problem. They're only essential to solving one puzzle, and due to creative laziness the reality-bending parallels and illusions in MGS2, it's exactly the same puzzle in both games.

Really, I find these weapons handy for just two things:

1) Beating Sniper Wolf the cheap, easy and safe way in MGS

2) Using the launcher as a melee weapon for one-hit KOs in MGS2.