It's a way that the English government can get more money from drivers. The name would suggest
that it is in place to reduce congestion but the exemption is based on exhaust emissions.
All electric cars do not have to pay this charge. At the time of writing (August 2013) there are no combustion engined vehicles that fall inside the set limits for free entry into the congestion zone.
These cars would have to be Euro 5 emissions compliant and produce less than 75g/km.
This has changed since 2010 when it was 100g/km + Euro5.
The Government introduced the ultra low emissions standards because so many cars were getting
into the congestion zone for free so the emissions were changed so the Government could make
more money to keep them in the manner they are used to and to pay for all the asylum seekers.
£10 a day if you drive in London between 07:00 and 18:00 on a weekday and pay the day before you go.
£12 if you pay after you have entered the congestion zone. £9 if you pre-pay with CC Autopay.

Smart Fortwo ED (Electric Drive) all models.
Yep, no smarts are free apart from the all electric version.
If you are from abroad, just drive in the CC zone without paying.
"Entry authorisation and penalties cannot be issued to non-UK numberplates"
So you don't have to pay, they won't chase or attempt to charge you.