Some free online resources for extra grammar explanations and exercises:

• French.about.com
• REPSIT – À la découverte du monde francophone (University of Calgary)
• Tex’s French Grammar (University of Texas, Austin)
• TV5 Monde: learning French (not just grammar), from the main French international TV channel > niveau A1 (débutant) & A2 (intermédiaire)
• Le Point du FLE: probably not as useful as the sites listed above, as it’s entirely in French, but yet more exercises…
• Meta-meta-medieval > outils utiles (Juliet O’Brien):
  yet more resources for French (with some overlap with the resources above), including links to various websites with yet more online exercises.
  NB: slightly out of date, links have not been checked recently.