While there are literally dozens, if not hundreds of free choices out there, you’ll want something that helps you with pronunciation and grammar, rather than just a list of vocabulary words, so we’re going to eliminate the text only sites.
is one of the biggest language sites on the Internet and I’ve used it myself, though not for Spanish. You can learn on a PC or on your phone, but you’ll need a mic. The program offers written and spoken language. The ratings as to how fluent you are seem to be a bit off, judging from what I’ve seen in Spanish, but it is probably one of the better choices for free.
Youtube has a number of videos that teach Spanish and these are great if you’re looking to speak and listen better. Find a video series that resonates with you and work from there.