Visas are available at most borders for all visitors except nationals from:

Africa: Angola, Benin, Botswana, Burkina Faso, Burundi, Cameroon, Central African Republic, Chad, Congo, Cote d'lviore, Djibouti, Eritrea, Ethiopia, Gabon, Gambia, Ghana, Guinea, Guinea Bissau, Guyana, Liberia, Madagascar, Mali, Mauritania, Mozambique, Niger, Nigeria, Senegal, Sierra Leone, Somalia, Sudan, Tanzania, Togo, Uganda, Zambia.

Asia: Afghanistan, Bangladesh, Cambodia, India, Iran, Laos, Lebanon, Mangolia, Pakistan, Papua New Guinea, Philippines, Russia, Sri Lanka, Vietnam. Europe:
Albania, Bosnia and Herzegovina, Macedonia.

South America: Belize, Colombia, Cuba.

In addition all Palestinians holding travel documents of whatsoever nationalities, and Moroccan females of (16-35) years old traveling alone.

Border visas are NOT available at the Allenby/King Hussein bridge crossing with Israel, so get one before hand.  These are obtainable from any Jordanian diplomatic mission abroad. If you leave Jordan at the Allenby/King Hussein bridge crossing with Israel, then return while the visa is still valid you do not need to buy a new one. This is because West Bank is recognized by Jordan as a part of Jordan.

Visas obtained in Jordanian consulates are valid for 3-4 months from the date of issue, and can be issued for multiple entries. Tourist visas allow a stay of up to one month initially. However, this period can easily be extended for up to another two months.