The Heroespedia now has an extensive spell section, courtesy of Ynhockey (descriptions) and Psychobabble (graphics). Check out the links under Magic School Specializations. The content is searchable, there are images of some of the spells, and best of all you can enter comments of your own.

Also, check out the new page for the Nightmare, which includes some tips for sieges. Be sure to keep an eye on the Recent changes page for more Heroes of Might and Magic IV information.