The most common mode of transport in Sri Lanka is via a three-wheeled automobile appropriately referred to as a three-wheeler (Tri-Shaw). Also known as Tuk-Tuks from the noise of their motors. These operate in a manner similar to taxis, and is a highly cost-efficient way to get around. However, three-wheelers have been linked to many illegal and criminal activities, But not common.
While it may be the most novel way to get around Colombo, it may not be the most cost efficient. Most Three-wheel drivers tend to over price foreigners, so never agree to the first estimate. The best price you can get is about Rs. 50 - 75 per Km for short jouneys and about Rs. 30 - 50 for long journeys ( more than 15 km). If you do come across a metered Tri-Shaw make sure the meter is switched on. But Taxi's are generally the same price and possibly safer. Having said that you probably have not experienced everything Sri lanka has to offer until you travel in one.