Duolingo.com is a free website where you can learn Spanish. Create an account with your email address, or log in via Facebook (this lets you connect and compete with your Facebook friends). Lessons let you practice reading, writing, listening, and speaking, and treat it like a game: lose lives when you make errors, earn experience points and "level up" as you master various vocabulary and grammar, and so on.
Duolingo has apps for Android and iPhone/iPad so you can learn and practice anywhere.