It's funny you should ask. I've been looking for one myself and plan to buy one soon. I can give you some tips that may help.
First of all, assuming that you're going to be getting an external that's USB-connected, it depends on whether you want to get a USB2.0 or USB3.0 connection. USB3.0 is faster, but at the same time it's newer tech and doesn't currently have many with a sufficient enough amount of customer reviews by which to get a thorough idea of what good they are as versus to USB2.0 connected externals which have been around longer and have a lot of customer reviews. It depends on which one you want to get.
If you are willing to sacrifice data transfer rates and go for a USB2.0, then I'd recommend a Western Digital Elements drive. Newegg.com has hundreds of customer reviews on each that get consistently about 4 out of 5 egg reviews and on amazon for at least 3.5 stars each. One on Newegg.com had 480+ reviews and 4 out of 5 eggs which I think are good odds.
However, if you want something faster and want to wing it with USB3.0, then I'd recommend what I'm going to be buying perhaps within the next week or so: Samsung Story Station 3.0. Surprisingly although it's only 5400 RPM, it has performance reviews that outspace 7200 RPM externals and are also USB3.0 such as the Western Digital MyBook and Seagate Freeagent models.
It all depends on what you need, what you want, and what you're willing to spend.