When I decide to learn Spanish on line I did a search and was quickly inundated with thousands of sights most of which ended up being irrelevant. However after a frustrating afternoon of trying to figure out which sites were actually helpful I came across one that seems to know what it is talking about and provides a wealth of information for free. LearnSpanish.com has been extremely helpful. Of course its ultimate goal is to sell you a product the free information is quite extensive and very well explained. There are numerous very good examples that are well explained and following the lessons there are free quizzes to test your knowledge.
There are numerous sites like this, and I am sure some of them are equally effective and well done. However most will want to sell you a product, probably a home study kit, and the information on the site is simply to help you enough to entice you into buying the kit.