The following passport holders can enter without a visa for up to 90 days.
Andorra, Antigua and Barbuda, Argentina, Australia, Austria, Bahamas, Bahrain, Barbados, Belgium, Belize, Brazil, Brunei, Bulgaria, Canada, Chile, Colombia, 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.
If you have a visa or residence permit from Canada, USA or a Schengen country you may enter wiithout a visa unless you are from Afghanistan, Albania, Algeria, Angola, Armenia, Bangladesh, Bosnia and Herzegovina, Botswana, Cameroon, China, Republic of the Congo, Democratic Republic of the Congo, Eritrea, Ethiopia, Ghana, Haiti, Hong Kong, Indonesia, Iran, Iraq, Jordan, Kenya, Laos, Lebanon, Liberia, Libya, Mali, Mongolia, Mozambique, Nepal, Nigeria, North Korea, Oman, Pakistan, Palestine, Sierra Leone, Somalia, Sri Lanka, Sudan, Syria, Timor-Leste, Vietnam and Yemen.
For everyone else you will have to make a visit to an embassy or consulate to arrange a visa.
Visa requirements for El Salvador
El Salvador embassies