Lenguaje.com - www.lenguaje.com
An excellent Spanish language site that includes tools for conjugation, phonetics, stems synonyms and antonyms, etc.
La Página del Idioma Español - www.elcastellano.org
A site dedicated to the Spanish language. There is grammar and vocabulary, but also discussion and debate about the Spanish language and its development.
Spanish Grammar Exercises - http://www.colby.edu/~bknelson/SLC/
A good place to practice and a strong resource to fine-tune your grammar.
Gramática y Ortografía - www.indiana.edu/~call/lengua.html
This site offers practice exercises, and also offers a variety of links to rules and other tools.
Conjugation Trainer - www.spaleon.com
A good site to practice your verb forms.
Conjugate Spanish Verbs Online - www.elemadrid.com/spanish_verbs.htm
This is an automated Spanish verb conjugator.
Aula Diez - www.auladiez.com
Practice your Spanish with interactive Spanish exercises such as writing, reading comprehension, multiple-choice, listening and association.