This is probably the greatest custom PC build of all time

My friends, I might have found the greatest custom PC ever conceived or constructed: the Lego Gaming Computer.
No, this isn't a computer designed specifically for the myriad Lego-themed games that have flooded the market over the last 10 years—though it certainly could do that with aplomb. The Lego Gaming Computer is a top-spec, completely-built-from-scratch X-shaped gaming rig that's made entirely out of Lego bricks.
Because ATX PCs aren't meant to be X-shaped, the mastermind behind the build—a guy called Mike Schropp—had to come up with some ingenious solutions, such as a custom-made GPU heatsink and a transverse-mounted motherboard that separated the GPU's airflow path from the other components.

But of course, the star of the show is the case itself, which stands about 50 centimetres tall (19 inches) and is built entirely out of Lego. No glue was used in its construction; instead, according to Schropp, "the build incorporates many advanced building techniques that greatly increase the strength of the system. Overlapping plates and bricks are used throughout the design, along with supports and beams to add stiffness. This results in a very strong system that can be easily be moved around, handled, picked up, and put down without any worry of causing damage."
While the case looks unwieldy, Schropp maintains that every component can be easily swapped out or upgraded. While it isn't clear from the photos, the case consists of two parts: the main chassis, which houses all of the computery bits, and then a cover that slips over the top. There are no screws holding the outer cover on.

There's so much awesome engineering in the Lego Gaming Computer that I don't really know where to start. In a normal upright desktop PC case, the CPU heatsink hangs free; in the Lego computer, the heatsink and fan are fully supported by Lego bricks. Likewise, thanks to the graphics card's unusual upright position, its mass is fully supported by Lego, so you don't have to worry about it wobbling around. The power supply has its own inlet and exhaust that's separate from the main airflow system. In total (I think!) there are six main Noctua system fans: three at the bottom of the case that pull air in, and three at the top for exhaust. That's in addition to the GPU, CPU, and other various fans used throughout.
The power supply started life as a small-form-factor Silverstone SFX 600W Gold unit, but Schropp took objection at Silverstone's use of cheap Taiwanese and Chinese capacitors—so he de-soldered them and swapped in some fancy Chemi-con Japanese caps. He also didn't like the placement of the PSU's cooling fan—so he removed it, cut some additional holes, and added a bunch of new Sunon fans. Schropp is dedicated.
There's probably a lot more that I haven't mentioned, but I think those are the highlights. Oh! The cable ducting is really neat, too.
I'm done gushing now. For more information, be sure to read Schropp's write-up of the build (it's thousands of words long). I should probably also mention that he'll build you a Lego Gaming Computer, too, but it isn't cheap: they start at about £1,100 ($1,600) for a Core i5/GTX 950 and go up from there. If you're willing to spend thousands, there are various customisations available as well. One day I'll buy one... one day.