If you aren't using Time Machine to back up your Mac, then it is time to start. Here are some options for a Time Machine backup drive.
Here are some recommendations of hard drives I have used:
I’ve used many Western Digital MyBooks in the past. Here is the current 1TB hard drive for less than $120.
For internal drives I’ve used several Western Digital Caviar Green 1TB 7200 RPM SATA II Drvies which run under $100.
I’ve also used those internal drives in a Vantec NexStar NST-D100SU 2.5-Inch/3.5-Inch SATA to USB 2.0 and eSATA Hard Drive Dock, which I have connected to my Mac via USB.
Currently I am using a Data Robotics DR04DD10 Drobo 4-Bays USB 2.0 and FireWire 800 Fully Automated SATA Robotic Storage Array for my time machine backups with four of those internal drives. It handles all of my Macs.
Another option is a portable drive. They are smaller and don’t require external power, but are a bit more expensive. I’ve used one similar to this in the past: Western Digital My Passport Essential 500 GB USB 2.0 Portable Hard Drive WDME5000TN