As of January 2011 only the nationals of the following non-EU/EFTA countries do not need a visa for entry into the Schengen Area: Albania*, Andorra, Antigua and Barbuda, Argentina, Australia, Bahamas, Barbados, Bosnia and Herzegovina*, Brazil, Brunei, Canada, Chile, Costa Rica, Croatia, El Salvador, Guatemala, Honduras, Israel, Japan, Macedonia*, Malaysia, Mauritius, Mexico, Monaco, Montenegro*, New Zealand, Nicaragua, Panama, Paraguay, Saint Kitts and Nevis, San Marino, Serbia*/**, Seychelles, Singapore, South Korea, Taiwan*** (Republic of China), United States, Uruguay, Vatican City, Venezuela, additionally persons holding British National (Overseas), Hong Kong SAR or Macau SAR passports. These visa-free visitors may not stay more than 90 days in a 180 day period and, in general, may not work.

Note that
while British subjects with the right of abode in the United Kingdom and British Overseas Territories citizens connected to Gibraltar are considered "United Kingdom nationals for European Union purposes" and therefore eligible for unlimited access to the Schengen Area,

British Overseas Territories citizens without the right of abode in the United Kingdom and British subjects without the right of abode in the United Kingdom as well as British Overseas citizens and British protected persons in general do require visas.

However, all British Overseas Territories citizens except those solely connected to the Cyprus Sovereign Base Areas are eligible for British citizenship and thereafter unlimited access to the Schengen Area.
Further note that
(*) nationals of Albania, Bosnia and Herzegovina, Macedonia, Montenegro and Serbia need a biometric passport to enjoy visa-free travel,
(**) Serbian nationals with passports issued by the Serbian Coordination Directorate (Serbs residing in Kosovo) still do need a visa and
(***) Taiwan nationals need their ID number to be stipulated in their passport to enjoy visa-free travel.

Citizens of Canada, Japan and the United States are permitted to work in Romania without the need to obtain a visa or any further authorisation for the period of their 90 day visa-free stay. However, this ability to work visa-free does not necessarily extend to other Schengen countries.

If you do need to obtain a visa from outside your own country, try obtaining it from somewhere else beside Budapest, where it can take 3 to 4 days. From Ljubljana the process can sometimes be done in a day because they are not so busy.