Can someone recommend a good external harddrive? What parameters should I take into account while choosing one (aside from the obvious, like storage capacity)?

Any help would be appreciated!

Speed would be my primary concern. There are few things you need to look in your HDD before buying one.
1.Seek time should to be <=10ms
2.Buffer size - More buffer size is better.Usual size can be from 4mb but make sure its not below it.
3.RPM - this determines the actual speed of rotation per minute.Get a drive with 7200rpm, although some come with 5200rpm.More rpm = data read/write happens faster.
The next thing is connectivity.When it comes to external HDDs , you'll need to choose carefully when selecting the connection types. If you have a firewire port on your pc, make sure the HDD has firewire connectivity or else stick with the USB type. Firewire is more preferred.If you travel a lot, then you definitely don't want the drive to start acting weird(like for eg. stop working while copying/reading data). So look for a drive which is durable. Such drives have extra coverings or silicon casing for more protection.If you do a lot of backing up, look for a hard drive with included back up and protection software.
The next thing would be choosing a brand that you easily recognize.Sure these may cost a little more than other substandard ones but its worth the cash. You don't want to have a cheaper drive made with inferior parts.
Some of the best HDDs out there are Western Digital, Seagate, Maxtor, Lacie.

Many thanks for that quick overview. Unfortunately my laptop doesn't support Firewire, so I'll have to go with the USB ones.

Get an SSD!!

Unfortunately SSDs tend to cost more than HDDs. And according to Register to see links, they have a limited lifespan. A big no-no for me.

Update: I'm still looking, haven't found a good one yet.