Here are two hard drives. They are Original Equipment Manufacturer so if you don't have any Serial ATA cables to connect it to your motherboard and power supply you will have to buy some.
Seagate Barracuda Model ST3500630AS 500 gigabyte Serial ATA Hard Drive - $105
New Egg Specials with Free Shipping (500GB and 750GB)
New Egg has a couple of good deals at the moment (with free shipping). It of course depends on your budget. I personally have 5 of the 500GB drives and love them.
Some of the posters on this forum have been talking about the poor quality control of Western Digital lately. They say the Western Digital Caviar Hard Drive models have had quite a few bad batches that got released on to the market. If you want a Western Digital Hard Drive go with the Raptor models.