If you've ever wondered what the top of Gordon Freeman's head looks like then Half-Life: Top-Down is the mod for you—it turns Half-Life into a top-down shooter, complete with plenty of camera customisation options.
You can watch it in action above, but I wouldn't judge it until you've seen the last 30 seconds, which shows how you can adjust the camera. On the default settings the overhead camera hugs the ceilings and, as the mod's creator points out, "it makes the camera get really close when the ceiling is low". It's a bit claustrophobic, and makes it hard to see where enemies are. But it's easily fixed thanks to a neat options menu that lets you manually set the height of the camera.
If you tell the camera that it doesn't have to stay in-bounds and bump the height up then it starts to look more like a conventional top-down shooter, and you can actually see the enemies you're fighting (see how it looks in the picture below). Unfortunately it will show everything that's out of bounds in a bright red colour, but I think it's worth putting up with. Beside, modder Sockman111 is working on a solution, perhaps by placing a big object with a texture far below.
The mod also tweaks the auto-aim to account for the fact you can't aim higher or lower on an enemy, and makes it easier to interact with objects if you aren't looking straight at them.
A word of warning: the creator says they're "not sure yet if the entire game is playable like this". One player in the comments of the mod has also reported a number of bugs. But it's something Sockman111 is working on, and most people seem pleased with the results.
You can grab it from ModDB.