One of the most complete, interactive grammar practice website can be found here. To find help in the writing of compositions, in English or Spanish, this site is really useful.
Other websites containing grammar exercises:
Learn Spanish: A good site with a wide array of exercises.
Spanish Flashcards: Check out this site if you are tired of making flashcards.
The Spanish Language Page: Cambridge University's Spanish links website.
Spanish Grammar Exercises: Helpful site with an abundance of exercises.
Sp-qz: A wide range of exercises, quizzes, and tests.
Wannalearn.com: Links to all things dealing with Spanish grammar.
Spanish Language Exercises: A good number of self-check exercises
Cela On-line Classes: Venezuelan page with advanced exercises.
Grammar Exercises Index: Numerous grammar exercises available.
Studyspanish.com: Tons of study aids of different sorts available.
Vocabulary Training Exercises: Site specializes in vocabulary and tense
Grammar Links: This site specializes in introductory grammar exercises.
Grammar Exercises: A site dedicated to developing grammar skills.
Spanish Language Resources: Site containing numerous introductory
Spanish Language Drills: 20 interactive grammar drills.