Language Resource Center

A lot of free lessons, tests, and quizzes, though it is membership based.

Lots of lessons that teach you Spanish grammar. There's also questions to test your knowledge. This website not only has this grammar section, but many more valuable resources.

a list of the most common Spanish verbs with full conjugations.

a guide with conjugations, drills and quizes organized by types of verbs.

a JavaScript-Tool to practice the different forms of the most important basic and irregular Spanish verbs.