London's black taxicabs - short for "taximeter cabriolets", that is, "cars with distance meters" - are a ubiquitous sight on the streets, and a common way to travel around the city in addition to the Tube and buses. In their favour, they're warm, comfortable and the drivers generally know where they're going, as they have to pass a rigorous test called "The Knowledge" before they can get their license to drive. The downside to the black cabs is that they're expensive, and unless you have very deep pockets there's no point using them for longer journeys across town. This situation has been exacerbated recently by Ken Livingstone's decision to let them charge more at night, since the drivers would often be reluctant to work at night.
Tip: although the traditional London taxi is black, they now often come in a multitude of colours. Try looking out for their distinctive shape, as in the photo above. That said, there are now newer, bigger, boxier models as well. If in doubt, look out for the orange "TAXI" light above the windscreen. If it's on, you can hail the cab. If not, you'll have to wait for another.
In addition to the official cabs, London also has many unlicensed "minicabs".
The official (and ugly) black cabs site is londonblackcabs.co.uk.