HGST, which is the new company owned by Western Digital that formerly was called Hitachi Global Storage Technologies, has recently announced the world's first 4TB Enterprise HDD: the Ultrastar 7K4000. This brings less power usage per gigabyte, a size large enough so that 2.4 Petabytes can fit in a regular 19-inch storage rack, and much more reliability to the table. The drive is an enterprise drive, which means it's meant for large datacenters or corporations that are moving toward "the cloud."
The drive is rated to 2-million hour MTBF (mean time between failure), which is much higher than the last generation's 1.2-million MTBF. It also gives 24% fewer watts per gigabyte, meaning less power consumption as well as lower heat. The 4TB size really speaks for itself, really, but I'll speak for it anyway. That size is just under the size of Wikipedia's raw dump back in January of 2010 (the dump size was 5.87 Terabytes).
Brendan Collins, vice president of product marketing at HGST, said,
There's no word on when we'll see consumer 4TB hard drives, but I'd expect to see some at least announced soon. You can read the full press release here.