Quia.com - Games that correspond directly to chapters in Realidades. (Find your book in the 'Textbook' drop down menu, then click 'submit'. Scroll down and find the chapter you wish to study from.)
Studyspanish.com - Excellent site with grammar explanation and practice quizzes for vocabulary and grammar.
Quizlet.com - Create your own flashcards OR practice spelling and listening to vocabulary words. OR Type in the name of our book and the chapter number to find vocabulary games and quizzes
Scholastic - a variety of games to practice grammar, vocabulary and more
Spanish Radio - Listen to Spanish radio! Select a radio station from anywhere in the Spanish speaking world! You may not understand everything you hear, but just listening will help you be comfortable with sounds of the language. Try out a Sports or music station. You may understand more than youthink!
About - large variety of games, explanations and exercises
BBC - a Great on line course for learning Spanish.
myHq - an extensive collection of links to various subjects to practice vocabulary and grammar
Conjuguemos - great self correcting fill in the blanks on MANY parts of grammar