Diccionario de la Lengua Española de la Real Academia:
An
authoritative Spanish dictionary (not a Spanish-English dictionary). To search a word, type in the
box at the top of the page (it looks a bit like a banner ad, so it
can be confusing)
Online conjugation quizzer: A simple but functional automatic conjugation quiz generator, written by Tomás. You can also take math quizzes in Spanish here to learn your numbers.