The most recent figures available at the moment are for August 2011. At this time the cheapest petrol in the world could be found in Venezuela where the average price was 2-3p per litre. This was way ahead even of Saudi Arabia, the world's largest oil producer, which was in second place at 8p. Libya, Turkmenistan and Bahrain followed - one little known fact about Turkmenistan is that every citizen is entitled to a free monthly petrol allowance of 120 litres. If you get through that, the average price per litre is 12, which still is pretty cheap.
All the remaining top 10 countries for good prices are Middle Eastern or North African states - you can see a full list here.

At this time the most expensive fuel in the world was in Norway - £1.65 a litre. Since then, of course, fuel prices have risen in most countries - in the UK it's around £1.34 at the time of writing.