The State of Qatar issues Tourist Visas on arrival for nationals of 33 countries:

Andorra, Australia, Austria, Belgium, Brunei, Canada, Denmark, Finland, France, Germany, Greece, Hong Kong, Iceland, Ireland, Italy, Japan, Liechtenstein, Luxemburg, Malaysia, Monaco, New Zealand, Norway, Portugal, San Marino, Spain, Sweden, Singapore, South Korea, Switzerland, The Netherlands, USA, UK, Vatican City.

This service allows travelers or hotels to apply for a visa online ahead of time. Applicants are required to supply a sponsor ID in the application, which can be from a relative living in Qatar or an ID supplied by a hotel.

Citizens from outside the above countries should arrange for a tourist visa through a Qatari embassy or hotel in Qatar.

Source: Hukoomi