Here are a few sites that offer free music... LEGALLY! (As far as I know...) It is usually not music that is popular, so it is just given out for free. This is often for promotional reasons.
Anyway, these are some sites that I snag free music from. I often end up finding songs I play over and over again. I hope you have the same luck.
This one is more popular for dance, dub step, and hip-hop sounds. Make sure you click songs that say "Free download" or something similar.
This has all different kinds of music from millions of users. Some of it is a little sketchy on the legality scale. However you can use your judgement in this area. Make sure you click songs that say "Free download" or something similar.
This site is very diverse and often advertises new upcoming artists. You can find many different types of music here. To actually get free music you need to check individual artists pages and see if any of the songs say "Free download" next to them. I look at the top download charts and go the pages of artists with songs that I like.
Amazon also offers a large amount of free music. You have to look hard, but I have found really good music .This is the direct link.
Otherwise, just check the free section on iTunes. They always have music that ends up being popular. I got "Animal" by Neon Trees, "Clarity" by Zedd (ft. Foxes), "Fireflies" by Owl City, and many other great songs for FREE!!!
Happy Hunting! Always keep your mind open when looking for new music!