If you think you can go to an electronics store and get a product for your mobile storage needs you'll experience a rude awakening. There isn't just one shelf with a handful external hard drives - Best Buy and Circuit City have racks full of storage products, and the e-tailers may have even larger selections.
It has become increasingly difficult to recommend a particular product, especially since the difference between two similar external hard drives is little. Most products use 3.5" desktop-type hard drives and offer at least a USB 2.0 interface. Access times and data transfer speeds will not vary much, as these are limited by the USB interface. Some products come with a Firewire IEEE1394 port, which can provide better performance; 1394b in particular is rated at 800 Mbit/s versus USB 2.0's 480 Mbit/s. So, if you need a simple external storage box for your everyday data, almost every product will do the job.
You will have to get an eSATA (external SATA) device if performance is critical for you. Thanks to this interface, eSATA devices perform as fast as if the hard drive were installed directly to a PC's Serial ATA controller. Another reason for purchasing more expensive storage devices is an enhanced feature set. This can be a comfortable backup solution that allows scheduling backups or synchronizing data between your external device and one or more PCs. Or you may want to buy into more expensive devices if portability and device dimensions are important for you. In this case we may have found a nice solution: Apricorn's $200 Aegis Mini offers 60 GB and comes in a tiny enclosure.