PicJumbo – “totally free photos for your commercial & personal works“. Categorized, but no search engine.
Gratisography – Free high-resolution pictures you can use on your personal and commercial projects. No search engine.
Pixabay – You can freely use any image from this website in digital and printed format, for personal and commercial use, without attribution requirement to the original author. Images are personally uploaded by contributors under Creative Commons Public Domain. The site functions beautifully and has great navigation, sorting, and searching functions.
Photopin offers free photographs for bloggers and creatives. Search returns photos as well as the proper attribution link. Searches Flickr Creative Commons photos.
Unsplash – Beautiful, hi res images. Click “Archive” to browse images or follow them on Tumblr. 10 new pictures every 10 days. No search function.
New Old Stock – Vintage photos from the public archives. Free of known copyright restrictions. No search function.