Hitachi, Seagate and Western Digital with the other boys to follow shortly
Western Digital green drive is good as static storage but is only 5400rpm and finds it slightly hard to keep up with the 7200 drive but wins hands down on the power consumption compared to the other two, the Hitachi Deskstar not good with server loads but handles small files with ease and wins on pricing. The Seagate is good with server loads but available as a standalone product which is a shame.
At the time of writing the cheapest 3TB was the Hitachi drive we could locate started at $195.00.
Important note installing 3TB drives may be difficult unless you run Windows 7. Check with your computer manufacturer to see if it can handle GPT partitions and whether your disk controller can work with 3TB disks.