http://spanish.about.com/
This is an excellent site with a wide variety of resources for Spanish students.
Has large numbers of lessons, information on culture and history, lists of false
cognates and improper verbs, help with tricky grammar and just about anything
else you could imagine yourself wanting.
http://www.spanishdict.com/
An online Spanish dictionary with over 56,000 entries and over 10,000 audio
files to help with pronunciation. Good for when the inaccuracy of online
translators becomes overwhelming.
http://www.aitech.ac.jp/~iteslj/v/s
This comprehensive page of quizzes is perfect for the steadily advancing student
desiring to enlarge his or her vocabulary.
Spanish Made
Easy (http://www.davidreilly.com/spanish/
This page, although small, is a good resource for people just starting to learn
Spanish. Includes a few lessons, comprised of mostly simple vocabulary and verb
use, and a word of the day feature.
http://jb002.tripod.com/languageA.htm
Another small site, this has short, simple lessons available for numerous
categories in grammar.
http://www.colby.edu/~bknelson/exercises/index.html
A professional-looking site dedicated to grammar exercises. Not as simple or as
specific as some of the others, but offers a wider variety of choices.
http://www.lingolex.com/spanish.htm
This site is somewhat lacking in organization, but there is a large amount of
information available including vocab lists, grammar lessons, and even an
automatic verb conjugation program.
http://www.studyspanish.com/
A tutorial site with lessons in increasing difficulty. This makes it useful to
students of all skill levels, although some material is restricted to paying
users only.
http://www.caslt.org/research/sp.htm
If there's anything that you couldn't find in one of the above links, this will
probably have what you are looking for.