1) Pexels - Pexels.com has so many high quality photos that you probably don't need to go anywhere else. I find 99% of my photos right here.
2) Kaboompics - Kaboompics has hundreds of royalty free photos to choose from. They are high quality and come in a variety of subjects.
3) Stocka.co - Stocka is relatively new, but has a decent number of stock photos to choose from. They are professional, by professional photographers, and royalty free.
4) Gratisography - These photos are all taken by the same photography and are beautiful. They're perfect if you're looking for a cohesive look for your site.
5) Start-up Stock Photos - This site is great for business bloggers or small businesses. All of the pictures are work related - chick on a computer, a Macbook, coffee on a desk, the kind of photography you see on this site.
What are your favorite stock photo sites?