Need to get a taxi to town from an unfamiliar airport? World Cab Fares gives you an estimate of the likely cost.
While the bare-bones but functional site doesn't specify its sources, the prices it offered for airports I was familiar with were all pretty much on the money. You can search via city name or airport code, and prices are offered in the local currency. While public transport will invariably be cheaper, if a taxi is the only option, it's helpful to be forewarned how much cash you'll need.