2 1/2 of the makers of the L4D(2) mod Dead Before Dawn went solo and created THE CORE.
It's a good 'mod' but it's lacking a lot of direction. Most of the puzzle rooms are just difficult because the rooms are rather misleading.
It looks great though the water textures seem to have been stretched badly. (I suggest you select the water surfaces and use the 'Fit' option, this will make the water look a lot better in-game because then the flow maps won't be resized to a size that small)
The GLaDoS voice sounds a bit better than the ones I've heard in other maps/mods. So that's a plus.