Pixabay
Free high-resolution stock photos for personal and commercial use. (Disclaimer: Usages licenses are sometimes not verified on free image sites, so use with caution.)
RBGStock
Free high-resolution photos for personal or commercial use. (Disclaimer: Usages licenses are sometimes not verified on free image sites, so use with caution.)
Unsplash
Free high-resolution stock photos for personal and commercial use. (Disclaimer: Usages licenses are sometimes not verified on free image sites, so use with caution.)
Creative Commons Search
Search engine for photos licensed under Creative Commons.
FreeImages
High-resolution stock photos for personal and commercial use. Mixture of free and "premium". Use "free photo categories" at bottom of page for categorized free images. (Disclaimer: Usages licenses are sometimes not verified on free image sites, so use with caution.)
Brands of the World
Great resource for finding vector copies of corporate logos. Quality varies, and sometimes logos are not the most recent version (so use with caution).