From 1 January 2012 Sri Lanka has introduced new visa rules for all foreign nationals visiting the country. 
According to the new rules, all visitors should obtain an electronic travel authorization (ETA) prior to their entry in the country. Only citizens of The Republic of Singapore and The Republic of Maldives are excempt from this new rule on the basis of reciprocity. The electronic travel authorization is valid initially for 30 days, but can be extended with up to 6 months. The ETA costs 10 USD for citizens of the SAARC countries, and 20 USD for all other nationals. For those who have not managed to secure the ETA in advance, there is an option to do that upon arrival upon 25 USD, but the service is limited (according to the official website). Read more on the official website.

For more information:

http://www.reisinformatieweblog.com/nieuwe-visum-regels-voor-sri-lanka/
http://www.eta.gov.lk/