I am currently using following 3 resources to learn Spanish. .
1. Duolingo - Available as Website, Android app, iPhone App.
2. Memrise - Available as Website, Android app, iPhone App.
3. BBC Languages : Mi Vida Loca - Available as Website only.
All these 3 resources have a lot in common -
1. They offer not just Spanish, but other major languages to learn as well.
2. As you will notice, they are *casual* for a casual learner like me :)
3. They are Free to use.
3. And they are pretty fun way to learn some language.
There are several paid resources as well. I will mention Babel among them. But I will go with these 3 for now.
And finally there is http://spanish.stackexchange.com/ for asking and answering questions relating to Spanish.