Unsplash describes itself thusly:
Beautiful, free photos. Gifted by the world’s most generous community of photographers. 🎁
And that’s exactly what it is: a massive (currently over 300,000) collection of completely free, high resolution images you can use for literally anything:
All photos published on Unsplash can be used for free. You can use them for commercial and noncommercial purposes. You do not need to ask permission from or provide credit to the photographer or Unsplash, although it is appreciated when possible.
If you’re curious about the history and motivation behind Unsplash, check out their manifesto. It’s an interesting read.
While its primarily a great source of images for projects, I also find Unsplash to be an invaluable resource for other reasons: Browsing the images is a great source of inspiration, many of the photos make for some stunning wallpaper, and you can extract some great color palettes from the collection.