Tuk-tuks are a quick, though noisy way to get around. Fares are usually 30-40 baht for a short hop (as of July 2012 it seems that the minimum has gone up to 40 baht for pre-arranged locations) and 50-100 baht for longer distances, depending on the proficiency of your bargaining. As a guide, expect to pay 40 baht from the old city to the riverside and Night Bazaar, 40-50 baht to the railway station, and 80-100 baht to the bus station or airport. Tuk-tuks parked near the bus and train stations will ask you for something like 120-150 baht. Just haggle or walk away to the nearest road and stop a passing tuk-tuk or songthaew there.
According to expats, the highest fee for a tuk-tuk at any time of day or night should be 150 baht for any location in town.