Georgia State University
Georgia State University: Hyper Physics: Neptunium
Good information on neptunium, all nicely arranged.
Web Elements
Web Elements Periodic Table: Neptunium
WebElements provides a very good source of basic information on neptunium, including physical properties, isotopes and electron configuration.
Corrosion Source
Corrosion Source: Neptunium
Easy to follow information on the history, sources, properties and isotopes of neptunium.