The following countries can enter without a visa.
Andorra, Antigua and Barbuda, Argentina, Australia, Austria, Bahamas, Bahrain, Barbados, Belgium, Belize, Brazil, Brunei, Bulgaria, Canada, Chile, Costa Rica, Croatia, Cyprus, Czech, Denmark, El Salvador, Estonia, Fiji, Finland, France, Germany, Greece, Guam, Guatemala, Honduras, Hungary, Iceland, India, Ireland, Israel, Italy, Japan, Korea(south), Kuwait, Latvia, Liechtenstein, Lithuania, Luxembourg, Macedonia, Madagascar, Malaysia, Malta, Marshall Islands, Mexico, Monaco, Netherlands, New Zealand, Nicaragua, Norway, Panama, Paraguay, Poland, Portugal, Qatar, Romania, Russia, Saint Vicent and Grenadines, Saint Lucia, San Kitts and Nevis, San Marino, Sao Tome and Principe, Saudi Arabia, Singapore, Slovakia, Slovenia, Solomon Islands, Spain, South Africa, Sweden, Switzerland, Taiwan, Trinidad and Tobago, Turkey, Tuvalu, Ukraine, UAE, UK, Uruguay, USA, Vanuatu and Vatican.
You may need to pay US$10 on arrival at the airport or a few dollars "tax" if crossing by land.
For everyone else you will have to make a visit to an embassy or consulate.
Visa requirements for El Salvador
El Salvador embassies
Last updated: 14 Oct 2013