As described here you now pay to enter London during the normal working week. This was done with a view to reducing the amount of traffic in and around the capital. Transport for London (TfL) report a reduction of just over 15% in traffic levels.
Well, I've noticed little or no reduction in traffic at all around the City. This isn't necessarily a Bad Thing, though, as the monies levied are apparently to be ploughed back into the public transport network around London. So, if there's not a great drop in traffic, they must be making a fortune...
According to their own figues, the system is taking �2.5 million a week though I've no idea what the running/capital costs are.
Related link: Official TfL Congestion Charge site